Reapply "Enable full type-checks in checked mode."

sdk/lib/_internal/compiler/implementation/dart_types.dart

Index: sdk/lib/_internal/compiler/implementation/dart_types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_types.dart b/sdk/lib/_internal/compiler/implementation/dart_types.dart
index 8a2cab29efcfe5a6d64d742572c7e4bdb6d86997..f77fe3a061f3e67c7061ef284a689a7eceddc180 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_types.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_types.dart
@@ -118,6 +118,8 @@ abstract class DartType {
    */
   TypeVariableType get typeVariableOccurrence => null;
 
+  void forEachTypeVariable(f(TypeVariableType variable)) {}
+
   TypeVariableType _findTypeVariableOccurrence(Link<DartType> types) {
     for (Link<DartType> link = types; !link.isEmpty ; link = link.tail) {
       TypeVariableType typeVariable = link.head.typeVariableOccurrence;
@@ -203,6 +205,10 @@ class TypeVariableType extends DartType {
 
   DartType get typeVariableOccurrence => this;
 
+  void forEachTypeVariable(f(TypeVariableType variable)) {
+    f(this);
+  }
+
   accept(DartTypeVisitor visitor, var argument) {
     return visitor.visitTypeVariableType(this, argument);
   }
@@ -407,6 +413,12 @@ abstract class GenericType extends DartType {
     return _findTypeVariableOccurrence(typeArguments);
   }
 
+  void forEachTypeVariable(f(TypeVariableType variable)) {
+    for (Link<DartType> link = typeArguments; !link.isEmpty; link = link.tail) {
+      link.head.forEachTypeVariable(f);
+    }
+  }
+
   void visitChildren(DartTypeVisitor visitor, var argument) {
     DartType.visitList(typeArguments, visitor, argument);
   }