Flow analysis: add assertions to AssignedVariables.declare.

These assertions verify that the client doesn't try to call `declare`
more than once on the same variable.  They should help avoid bugs as I
work on integrating the logic for variable patterns more deeply with
flow analyis.

Change-Id: Ic7ffec8bd5f542cef0067a5308d86de6e48ec8ea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/274521
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/_fe_analyzer_shared/lib/src/type_inference/assigned_variables.dart

@@ -75,11 +75,19 @@ class AssignedVariables<Node extends Object, Variable extends Object> {
   ///
   /// It is not required for the declaration to be seen prior to its use (this
   /// is to allow for error recovery in the analyzer).
-  void declare(Variable variable) {
+  ///
+  /// By default, this method contains assertions to make sure the client
+  /// doesn't call [declare] more than once on the same variable.  However,
+  /// there are some situations where it is difficult for the client to avoid
+  /// this, so the check can be disabled by passing `true` for
+  /// [ignoreDuplicates].
+  void declare(Variable variable, {bool ignoreDuplicates = false}) {
     assert(!_isFinished);
     int variableKey = promotionKeyStore.keyForVariable(variable);
-    _stack.last.declared.add(variableKey);
-    anywhere.declared.add(variableKey);
+    bool newlyDeclared = _stack.last.declared.add(variableKey);
+    assert(ignoreDuplicates || newlyDeclared);
+    newlyDeclared = anywhere.declared.add(variableKey);
+    assert(ignoreDuplicates || newlyDeclared);
   }
 
   /// This method may be called during pre-traversal, to mark the end of a
@@ -171,8 +179,11 @@ class AssignedVariables<Node extends Object, Variable extends Object> {
       assert(_stack.length == 1, "Unexpected stack: $_stack");
       AssignedVariablesNodeInfo last = _stack.last;
       Set<int> undeclaredReads = last.read.difference(last.declared);
-      assert(undeclaredReads.isEmpty,
-          'Variables read from but not declared: $undeclaredReads');
+      List<Variable?> undeclaredReadVars = [
+        for (int key in undeclaredReads) promotionKeyStore.variableForKey(key)
+      ];
+      assert(undeclaredReadVars.isEmpty,
+          'Variables read from but not declared: $undeclaredReadVars');
       Set<int> undeclaredWrites = last.written.difference(last.declared);
       assert(undeclaredWrites.isEmpty,
           'Variables written to but not declared: $undeclaredWrites');

pkg/analyzer/lib/src/dart/resolver/flow_analysis_visitor.dart

@@ -694,16 +694,18 @@ class _AssignedVariablesVisitor extends RecursiveAstVisitor<void> {
           member.expression.accept(this);
         } else if (member is SwitchPatternCaseImpl) {
           var guardedPattern = member.guardedPattern;
-          assignedVariables.beginNode();
+          guardedPattern.pattern.accept(this);
           for (var variable in guardedPattern.variables.values) {
             assignedVariables.declare(variable);
           }
           guardedPattern.whenClause?.accept(this);
-          assignedVariables.endNode(node);
         }
       }
       for (var variable in group.variables.values) {
-        assignedVariables.declare(variable);
+        // We pass `ignoreDuplicates: true` because this variable might be the
+        // same as one of the variables declared earlier under a specific switch
+        // case.
+        assignedVariables.declare(variable, ignoreDuplicates: true);
       }
       group.statements.accept(this);
     }

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -5071,7 +5071,9 @@ class BodyBuilder extends StackListenerImpl
       }
     }
     push(parameter);
-    typeInferrer.assignedVariables.declare(variable);
+    // We pass `ignoreDuplicates: true` because the variable might have been
+    // previously passed to `declare` in the `BodyBuilder` constructor.
+    typeInferrer.assignedVariables.declare(variable, ignoreDuplicates: true);
   }
 
   @override