Add more tests and fix the parser to deal with generic superclasses for mixin applications.

sdk/lib/_internal/compiler/implementation/scanner/parser.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/parser.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/parser.dart b/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
index 06ef26596c3e78c2898447adfd11f855e41ad3cb..ef0f4174df60b630e8d2e16f7d3970609700f8ed 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/parser.dart
@@ -490,7 +490,7 @@ class Parser {
     Token extendsKeyword;
     if (optional('extends', token)) {
       extendsKeyword = token;
-      if (optional('with', token.next.next)) {
+      if (optional('with', peekAfterType(token.next))) {
         token = parseMixinApplication(token.next);
       } else {
         token = parseType(token.next);