Add type inference logic for map literals

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 1337 matching lines @@
     debugEvent("LiteralNull");
     push(new KernelNullLiteral()..fileOffset = offsetForToken(token));
   }
 
   @override
   void handleLiteralMap(
       int count, Token beginToken, Token constKeyword, Token endToken) {
     debugEvent("LiteralMap");
     List<MapEntry> entries = popList(count) ?? <MapEntry>[];
     List<DartType> typeArguments = pop();
-    DartType keyType = const DynamicType();
-    DartType valueType = const DynamicType();
+    DartType keyType;
+    DartType valueType;
     if (typeArguments != null) {
       if (typeArguments.length != 2) {
-        keyType = const DynamicType();
-        valueType = const DynamicType();
+        keyType = null;
+        valueType = null;
         warningNotError(
             "Map literal requires two type arguments.", beginToken.charOffset);
       } else {
         keyType = typeArguments[0];
         valueType = typeArguments[1];
       }
     }
     push(new KernelMapLiteral(entries,
         keyType: keyType, valueType: valueType, isConst: constKeyword != null)
       ..fileOffset = constKeyword?.charOffset ?? offsetForToken(beginToken));
@@ skipping 1823 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }