diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index 1aaf1925f69..9ce1b2c4473 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -516,7 +516,7 @@ static void SetupForGenericSnapshotCreation() {
 
 
 int main(int argc, char** argv) {
-  const int EXTRA_VM_ARGUMENTS = 1;
+  const int EXTRA_VM_ARGUMENTS = 2;
   CommandLineOptions vm_options(argc + EXTRA_VM_ARGUMENTS);
 
   // Initialize the URL mapping array.
@@ -536,7 +536,9 @@ int main(int argc, char** argv) {
   DartUtils::SetOriginalWorkingDirectory();
 
   vm_options.AddArgument("--load_deferred_eagerly");
-
+  // Workaround until issue 21620 is fixed.
+  // (https://github.com/dart-lang/sdk/issues/21620)
+  vm_options.AddArgument("--no-concurrent_sweep");
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());
 
   // Initialize the Dart VM.
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index c3c152b8abf..46f3f4dcbaa 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -2332,6 +2332,9 @@ void ClassFinalizer::FinalizeClass(const Class& cls) {
   if (cls.is_finalized()) {
     return;
   }
+  if (FLAG_trace_class_finalization) {
+    OS::Print("Finalize %s\n", cls.ToCString());
+  }
   if (cls.is_patch()) {
     // The fields and functions of a patch class are copied to the
     // patched class after parsing. There is nothing to finalize.
@@ -2340,9 +2343,6 @@ void ClassFinalizer::FinalizeClass(const Class& cls) {
     cls.set_is_finalized();
     return;
   }
-  if (FLAG_trace_class_finalization) {
-    OS::Print("Finalize %s\n", cls.ToCString());
-  }
   if (cls.IsMixinApplication()) {
     // Copy instance methods and fields from the mixin class.
     // This has to happen before the check whether the methods of
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index dfe2c697d1d..80902546206 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -344,6 +344,12 @@ RawError* Compiler::CompileClass(const Class& cls) {
       ClassFinalizer::FinalizeClass(parse_class);
       parse_class.reset_is_marked_for_parsing();
     }
+    for (intptr_t i = (patch_list.Length() - 1); i >=0 ; i--) {
+      parse_class ^= patch_list.At(i);
+      ASSERT(!parse_class.IsNull());
+      ClassFinalizer::FinalizeClass(parse_class);
+      parse_class.reset_is_marked_for_parsing();
+    }
 
     return Error::null();
   } else {
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index 9562941a55c..cb5d584d94d 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -71,6 +71,46 @@ const char* CanonicalFunction(const char* func) {
 }
 
 
+#if defined(DEBUG)
+// An object visitor which will iterate over all the function objects in the
+// heap and check if the result type and parameter types are canonicalized
+// or not. An assertion is raised if a type is not canonicalized.
+class FunctionVisitor : public ObjectVisitor {
+ public:
+  explicit FunctionVisitor(Isolate* isolate) :
+      ObjectVisitor(isolate),
+      classHandle_(Class::Handle(isolate)),
+      funcHandle_(Function::Handle(isolate)),
+      typeHandle_(AbstractType::Handle(isolate)) {}
+
+  void VisitObject(RawObject* obj) {
+    if (obj->IsFunction()) {
+      funcHandle_ ^= obj;
+      classHandle_ ^= funcHandle_.Owner();
+      // Verify that the result type of a function is canonical or a
+      // TypeParameter.
+      typeHandle_ ^= funcHandle_.result_type();
+      ASSERT(typeHandle_.IsNull() ||
+             typeHandle_.IsTypeParameter() ||
+             typeHandle_.IsCanonical());
+      // Verify that the types in the function signature are all canonical or
+      // a TypeParameter.
+      const intptr_t num_parameters = funcHandle_.NumParameters();
+      for (intptr_t i = 0; i < num_parameters; i++) {
+        typeHandle_ = funcHandle_.ParameterTypeAt(i);
+        ASSERT(typeHandle_.IsTypeParameter() || typeHandle_.IsCanonical());
+      }
+    }
+  }
+
+ private:
+  Class& classHandle_;
+  Function& funcHandle_;
+  AbstractType& typeHandle_;
+};
+#endif  // #if defined(DEBUG).
+
+
 static RawInstance* GetListInstance(Isolate* isolate, const Object& obj) {
   if (obj.IsInstance()) {
     const Library& core_lib = Library::Handle(Library::CoreLibrary());
@@ -1463,6 +1503,12 @@ DART_EXPORT Dart_Handle Dart_CreateSnapshot(
   if (::Dart_IsError(state)) {
     return state;
   }
+  isolate->heap()->CollectAllGarbage();
+#if defined(DEBUG)
+  FunctionVisitor check_canonical(isolate);
+  isolate->heap()->VisitObjects(&check_canonical);
+#endif  // #if defined(DEBUG).
+
   // Since this is only a snapshot the root library should not be set.
   isolate->object_store()->set_root_library(Library::Handle(isolate));
   FullSnapshotWriter writer(vm_isolate_snapshot_buffer,
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index 57aca2be4a0..0c69b39f82d 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -393,6 +393,9 @@ class RawObject {
   bool IsScript() {
     return ((GetClassId() == kScriptCid));
   }
+  bool IsFunction() {
+    return ((GetClassId() == kFunctionCid));
+  }
 
   intptr_t Size() const {
     uword tags = ptr()->tags_;
diff --git a/runtime/vm/snapshot_test.cc b/runtime/vm/snapshot_test.cc
index 5c9a28a5d3c..e4f682946d0 100644
--- a/runtime/vm/snapshot_test.cc
+++ b/runtime/vm/snapshot_test.cc
@@ -20,6 +20,7 @@ namespace dart {
 
 DECLARE_FLAG(bool, enable_type_checks);
 DECLARE_FLAG(bool, load_deferred_eagerly);
+DECLARE_FLAG(bool, concurrent_sweep);
 
 // Check if serialized and deserialized objects are equal.
 static bool Equals(const Object& expected, const Object& actual) {
@@ -1235,7 +1236,10 @@ UNIT_TEST_CASE(ScriptSnapshot) {
 
   bool saved_load_deferred_eagerly_mode = FLAG_load_deferred_eagerly;
   FLAG_load_deferred_eagerly = true;
-
+  // Workaround until issue 21620 is fixed.
+  // (https://github.com/dart-lang/sdk/issues/21620)
+  bool saved_concurrent_sweep_mode = FLAG_concurrent_sweep;
+  FLAG_concurrent_sweep = false;
   {
     // Start an Isolate, and create a full snapshot of it.
     TestIsolateScope __test_isolate__;
@@ -1251,8 +1255,8 @@ UNIT_TEST_CASE(ScriptSnapshot) {
     memmove(full_snapshot, isolate_snapshot, isolate_snapshot_size);
     Dart_ExitScope();
   }
-
   FLAG_load_deferred_eagerly = saved_load_deferred_eagerly_mode;
+  FLAG_concurrent_sweep = saved_concurrent_sweep_mode;
 
   // Test for Dart_CreateScriptSnapshot.
   {
@@ -1379,6 +1383,8 @@ UNIT_TEST_CASE(ScriptSnapshot1) {
 
   bool saved_load_deferred_eagerly_mode = FLAG_load_deferred_eagerly;
   FLAG_load_deferred_eagerly = true;
+  bool saved_concurrent_sweep_mode = FLAG_concurrent_sweep;
+  FLAG_concurrent_sweep = false;
   {
     // Start an Isolate, and create a full snapshot of it.
     TestIsolateScope __test_isolate__;
@@ -1394,6 +1400,7 @@ UNIT_TEST_CASE(ScriptSnapshot1) {
     memmove(full_snapshot, isolate_snapshot, isolate_snapshot_size);
     Dart_ExitScope();
   }
+  FLAG_concurrent_sweep = saved_concurrent_sweep_mode;
 
   {
     // Create an Isolate using the full snapshot, load a script and create
@@ -1469,7 +1476,8 @@ UNIT_TEST_CASE(ScriptSnapshot2) {
   FLAG_enable_type_checks = false;
   bool saved_load_deferred_eagerly_mode = FLAG_load_deferred_eagerly;
   FLAG_load_deferred_eagerly = true;
-
+  bool saved_concurrent_sweep_mode = FLAG_concurrent_sweep;
+  FLAG_concurrent_sweep = false;
   {
     // Start an Isolate, and create a full snapshot of it.
     TestIsolateScope __test_isolate__;
@@ -1485,6 +1493,7 @@ UNIT_TEST_CASE(ScriptSnapshot2) {
     memmove(full_snapshot, isolate_snapshot, isolate_snapshot_size);
     Dart_ExitScope();
   }
+  FLAG_concurrent_sweep = saved_concurrent_sweep_mode;
 
   {
     // Create an Isolate using the full snapshot, load a script and create