Use for-in loop variable name if possible

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

 // Copyright (c) 2016, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart';
@@ skipping 793 matching lines @@
       // Find a type for the element. Use the element type of the indexer of the
       // array, as this is stronger than the iterator's `get current` type, for
       // example, `get current` includes null.
       // TODO(sra): The element type of a container type mask might be better.
       TypeMask type = astAdapter.inferredIndexType(forInStatement);
 
       HInstruction index = localsHandler.readLocal(indexVariable);
       HInstruction value = new HIndex(array, index, null, type);
       add(value);
 
-      localsHandler.updateLocal(
-          astAdapter.getLocal(forInStatement.variable), value);
+      Local loopVariableLocal = astAdapter.getLocal(forInStatement.variable);
+      localsHandler.updateLocal(loopVariableLocal, value);
+      // Hint to name loop value after name of loop variable.
+      if (loopVariableLocal is !SyntheticLocal) {
+        value.sourceElement ??= loopVariableLocal;
+      }
 
       forInStatement.body.accept(this);
     }
 
     void buildUpdate() {
       // See buildBody as to why we check here.
       buildConcurrentModificationErrorCheck();
 
       // TODO(sra): It would be slightly shorter to generate `a[i++]` in the
       // body (and that more closely follows what an inlined iterator would do)
@@ skipping 35 matching lines @@
       TypeMask mask = astAdapter.typeOfIteratorMoveNext(forInStatement);
       _pushDynamicInvocation(forInStatement, mask, <HInstruction>[iterator],
           selector: Selectors.moveNext);
       return popBoolified();
     }
 
     void buildBody() {
       TypeMask mask = astAdapter.typeOfIteratorCurrent(forInStatement);
       _pushDynamicInvocation(forInStatement, mask, [iterator],
           selector: Selectors.current);
-      localsHandler.updateLocal(
-          astAdapter.getLocal(forInStatement.variable), pop());
+      Local loopVariableLocal = astAdapter.getLocal(forInStatement.variable);
+      HInstruction value = pop();
+      localsHandler.updateLocal(loopVariableLocal, value);
+      // Hint to name loop value after name of loop variable.
+      if (loopVariableLocal is !SyntheticLocal) {
+        value.sourceElement ??= loopVariableLocal;
+      }
       forInStatement.body.accept(this);
     }
 
     loopHandler.handleLoop(
         forInStatement, buildInitializer, buildCondition, () {}, buildBody);
   }
 
   void _buildAsyncForIn(ir.ForInStatement forInStatement) {
     // The async-for is implemented with a StreamIterator.
     HInstruction streamIterator;
@@ skipping 2223 matching lines @@
     enterBlock.setBlockFlow(
         new HTryBlockInformation(
             kernelBuilder.wrapStatementGraph(bodyGraph),
             exception,
             kernelBuilder.wrapStatementGraph(catchGraph),
             kernelBuilder.wrapStatementGraph(finallyGraph)),
         exitBlock);
     kernelBuilder.inTryStatement = previouslyInTryStatement;
   }
 }