Add type inference logic for "for-in" loops.

pkg/front_end/lib/src/fasta/kernel/body_builder.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.
 
 library fasta.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, MemberKind, optional;
@@ skipping 2224 matching lines @@
     Expression expression = popForValue();
     var lvalue = pop();
     exitLocalScope();
     JumpTarget continueTarget = exitContinueTarget();
     JumpTarget breakTarget = exitBreakTarget();
     if (continueTarget.hasUsers) {
       body = new LabeledStatement(body);
       continueTarget.resolveContinues(body);
     }
     VariableDeclaration variable;
+    bool declaresVariable = false;
     if (lvalue is VariableDeclaration) {
+      declaresVariable = true;
       variable = lvalue;
     } else if (lvalue is FastaAccessor) {
       /// We are in this case, where `lvalue` isn't a [VariableDeclaration]:
       ///
       ///     for (lvalue in expression) body
       ///
       /// This is normalized to:
       ///
       ///     for (final #t in expression) {
       ///       lvalue = #t;
       ///       body;
       ///     }
       variable = new VariableDeclaration.forValue(null);
       body = combineStatements(
           new KernelExpressionStatement(lvalue
               .buildAssignment(new VariableGet(variable), voidContext: true)),
           body);
     } else {
       variable = new VariableDeclaration.forValue(buildCompileTimeError(
           "Expected lvalue, but got ${lvalue}", forToken.next.next.charOffset));
     }
-    Statement result = new ForInStatement(variable, expression, body,
+    Statement result = new KernelForInStatement(
+        variable, expression, body, declaresVariable,
         isAsync: awaitToken != null)
       ..fileOffset = body.fileOffset;
     if (breakTarget.hasUsers) {
       result = new LabeledStatement(result);
       breakTarget.resolveBreaks(result);
     }
     exitLoopOrSwitch(result);
   }
 
   @override
@@ skipping 919 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }