Only attempt to do type inference on fields that lack a declared type.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
index ee90588e0fec5d36b0c47bda0f3bdb4b057a0b46..c6f206933384f2afa355c9da30b58ad13d0ce9ef 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_field_builder.dart
@@ -34,16 +34,22 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
   final Field field;
   final List<MetadataBuilder> metadata;
   final KernelTypeBuilder type;
-  final Token initializerToken;
-
-  KernelFieldBuilder(this.metadata, this.type, String name, int modifiers,
-      Builder compilationUnit, int charOffset, this.initializerToken)
+  final Token initializerTokenForInference;
+  final bool hasInitializer;
+
+  KernelFieldBuilder(
+      this.metadata,
+      this.type,
+      String name,
+      int modifiers,
+      Builder compilationUnit,
+      int charOffset,
+      this.initializerTokenForInference,
+      this.hasInitializer)
       : field = new KernelField(null, fileUri: compilationUnit?.relativeFileUri)
           ..fileOffset = charOffset,
         super(name, modifiers, compilationUnit, charOffset);
 
-  bool get hasInitializer => initializerToken != null;
-
   void set initializer(Expression value) {
     if (!hasInitializer && value is! NullLiteral && !isConst && !isFinal) {
       internalError("Attempt to set initializer on field without initializer.");
@@ -63,7 +69,9 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
       ..hasImplicitGetter = isInstanceMember
       ..hasImplicitSetter = isInstanceMember && !isConst && !isFinal
       ..isStatic = !isInstanceMember;
-    if (initializerToken != null && !initializerToken.isEof) {
+    if (initializerTokenForInference != null &&
+        !initializerTokenForInference.isEof) {
+      assert(type == null);
       library.loader.typeInferenceEngine.recordField(field);
     }
     return field;
@@ -74,7 +82,8 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
   @override
   void prepareInitializerInference(
       SourceLibraryBuilder library, ClassBuilder currentClass) {
-    if (initializerToken != null && !initializerToken.isEof) {
+    if (initializerTokenForInference != null &&
+        !initializerTokenForInference.isEof) {
       var memberScope =
           currentClass == null ? library.scope : currentClass.scope;
       // TODO(paulberry): Is it correct to pass library.uri into BodyBuilder, or
@@ -95,7 +104,7 @@ class KernelFieldBuilder extends FieldBuilder<Expression> {
           library.uri,
           typeInferrer);
       Parser parser = new Parser(bodyBuilder);
-      Token token = parser.parseExpression(initializerToken);
+      Token token = parser.parseExpression(initializerTokenForInference);
       Expression expression = bodyBuilder.popForValue();
       bodyBuilder.checkEmpty(token.charOffset);
       initializer = expression;