Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/analyzer/mock_type.dart

Index: pkg/front_end/lib/src/fasta/analyzer/mock_type.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/mock_type.dart b/pkg/front_end/lib/src/fasta/analyzer/mock_type.dart
index ce34190cd9d8900ff910e3078d3d10a92988ade4..396ec1aab6d381a2c44efe1b53e0b3f1af927237 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/mock_type.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/mock_type.dart
@@ -59,6 +59,8 @@ abstract class MockType extends DartType {
 }
 
 abstract class MockInterfaceType extends MockType implements InterfaceType {
+  ClassElement get element => internalError("not supported.");
+
   List<PropertyAccessorElement> get accessors => internalError("not supported.");
 
   List<ConstructorElement> get constructors => internalError("not supported.");
@@ -108,6 +110,11 @@ abstract class MockInterfaceType extends MockType implements InterfaceType {
   PropertyAccessorElement lookUpSetterInSuperclass(
       String name, LibraryElement library) => internalError("not supported.");
 
+  InterfaceType instantiate(List<DartType> argumentTypes) => internalError("not supported.");
+
+  InterfaceType substitute2(
+      List<DartType> argumentTypes, List<DartType> parameterTypes) => internalError("not supported.");
+
   InterfaceType substitute4(List<DartType> argumentTypes) => internalError("not supported.");
 
   get isDartCoreNull => internalError("not supported.");