[ VM ] Ignore contexts which have been optimized out during stack overflow checks

Fixes https://github.com/dart-lang/sdk/issues/38182

Change-Id: I2b042b9a320e16fc88bee6ea2c0c93598e574c00
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115880
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Aart Bik <ajcbik@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

runtime/tests/vm/dart/regress_38182_test.dart

@@ -0,0 +1,35 @@
+// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// VMOptions=--stacktrace_every=1 --deterministic
+
+void foo1(par) {
+  try {
+    () {
+      // The parameter `par` has to be captured within a closure, but it doesn't
+      // matter whether or not it's actually used.
+      print(par.runtimeType);
+    };
+    // We need to throw, otherwise the crash doesn't happen. We don't need to
+    // catch it explicitly, however.
+    throw '';
+  } finally {
+    // We need to trigger a lot of stack overflow checks. Somewhere around
+    // 20000 seems to work.
+    int x = 0;
+    for (int loc1 = 0; loc1 < 20000; loc1++) {
+      x += loc1;
+    }
+    print(x);
+  }
+}
+
+main() {
+  try {
+    // Parameter isn't important.
+    foo1(null);
+  } catch (e) {
+    print('foo1 threw');
+  }
+}

runtime/vm/debugger.cc

@@ -1156,7 +1156,8 @@ const Context& ActivationFrame::GetSavedCurrentContext() {
       } else if (obj.IsContext()) {
         ctx_ = Context::Cast(obj).raw();
       } else {
-        ASSERT(obj.IsNull());
+        ASSERT(obj.IsNull() || obj.raw() == Symbols::OptimizedOut().raw());
+        ctx_ = Context::null();
       }
       return ctx_;
     }
@@ -1431,7 +1432,12 @@ RawObject* ActivationFrame::GetRelativeContextVar(intptr_t var_ctx_level,
                                                   intptr_t ctx_slot,
                                                   intptr_t frame_ctx_level) {
   const Context& ctx = GetSavedCurrentContext();
-  ASSERT(!ctx.IsNull());
+
+  // It's possible that ctx was optimized out as no locals were captured by the
+  // context. See issue #38182.
+  if (ctx.IsNull()) {
+    return Symbols::OptimizedOut().raw();
+  }
 
   intptr_t level_diff = frame_ctx_level - var_ctx_level;
   if (level_diff == 0) {