Fix type annotations that violate subtyping constraint on overriden methods.

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index 5a60a7215535baa7b10e5df1e4e9e30bdcd35d57..713cbc1414d0e4859fe0fa6d08bb3a2e92da74eb 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -218,7 +218,7 @@ class TypeVariableType extends DartType {
 
   DartType unalias(Compiler compiler) => this;
 
-  DartType get typeVariableOccurrence => this;
+  TypeVariableType get typeVariableOccurrence => this;
 
   void forEachTypeVariable(f(TypeVariableType variable)) {
     f(this);
@@ -661,7 +661,7 @@ class FunctionType extends DartType {
 
   DartType unalias(Compiler compiler) => this;
 
-  DartType get typeVariableOccurrence {
+  TypeVariableType get typeVariableOccurrence {
     TypeVariableType typeVariableType = returnType.typeVariableOccurrence;
     if (typeVariableType != null) return typeVariableType;