More tests for creating undefined methods and functions.

pkg/analysis_testing/lib/mock_sdk.dart

Index: pkg/analysis_testing/lib/mock_sdk.dart
diff --git a/pkg/analysis_testing/lib/mock_sdk.dart b/pkg/analysis_testing/lib/mock_sdk.dart
index 99cf23c15e109e8a2e8cc230b2fa0efb911c4b98..224d4384a90399c9ec9333af81c373b0c8b75238 100644
--- a/pkg/analysis_testing/lib/mock_sdk.dart
+++ b/pkg/analysis_testing/lib/mock_sdk.dart
@@ -26,9 +26,15 @@ class MockSdk implements DartSdk {
           class Symbol {}
           class Type {}
 
-          class String extends Object {}
+          abstract class Comparable<T> {
+            int compareTo(T other);
+          }
+
+          class String implements Comparable<String> {
+          }
+
           class bool extends Object {}
-          abstract class num extends Object {
+          abstract class num implements Comparable<num> {
             num operator +(num other);
             num operator -(num other);
             num operator *(num other);