Analyze locals in await for loops as inside a try-finally.

pkg/compiler/lib/src/ssa/builder.dart

 // Copyright (c) 2012, 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.
 
 part of ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   SsaCodeGeneratorTask generator;
   SsaBuilderTask builder;
   SsaOptimizerTask optimizer;
@@ skipping 6004 matching lines @@
   visitSwitchCase(ast.SwitchCase node) {
     compiler.internalError(node, 'SsaFromAstMixin.visitSwitchCase.');
   }
 
   visitCaseMatch(ast.CaseMatch node) {
     compiler.internalError(node, 'SsaFromAstMixin.visitCaseMatch.');
   }
 
   /// Calls [buildTry] inside a synthetic try block with [buildFinally] in the
   /// finally block.
+  ///
+  /// Note that to get the right locals behavior, the code visited by [buildTry]
+  /// and [buildFinally] must have been analyzed as if inside a try-statement by
+  /// [ClosureTranslator].
   void buildProtectedByFinally(void buildTry(), void buildFinally()) {
+    // Save the current locals. The finally block must not reuse the existing
+    // locals handler. None of the variables that have been defined in the
+    // body-block will be used, but for loops we will add (unnecessary) phis
+    // that will reference the body variables. This makes it look as if the
+    // variables were used in a non-dominated block.
     HBasicBlock enterBlock = openNewBlock();
     HTry tryInstruction = new HTry();
     close(tryInstruction);
     bool oldInTryStatement = inTryStatement;
     inTryStatement = true;
 
     HBasicBlock startTryBlock;
     HBasicBlock endTryBlock;
     HBasicBlock startFinallyBlock;
     HBasicBlock endFinallyBlock;
@@ skipping 891 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }