Implement least upper bound.

tests/compiler/dart2js/resolver_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import 'dart:collection';
 
 import "../../../sdk/lib/_internal/compiler/implementation/resolution/resolution.dart";
@@ skipping 628 matching lines @@
   Expect.equals(expectedElementCount, map(visitor).length);
 
   compareWarningKinds(script, expectedWarnings, compiler.warnings);
   compareWarningKinds(script, expectedErrors, compiler.errors);
   compareWarningKinds(script, expectedInfos, compiler.infos);
 }
 
 testClassHierarchy() {
   final MAIN = "main";
   MockCompiler compiler = new MockCompiler();
-  compiler.parseScript("""class A extends B {}
-                          class B extends A {}
+  compiler.parseScript("""class A extends A {}
                           main() { return new A(); }""");
   FunctionElement mainElement = compiler.mainApp.find(MAIN);
   compiler.resolver.resolve(mainElement);
   Expect.equals(0, compiler.warnings.length);
+  Expect.equals(1, compiler.errors.length);
+  Expect.equals(MessageKind.CYCLIC_CLASS_HIERARCHY,
+                compiler.errors[0].message.kind);
+
+  compiler = new MockCompiler();
+  compiler.parseScript("""class A extends B {}
+                          class B extends A {}
+                          main() { return new A(); }""");
+  mainElement = compiler.mainApp.find(MAIN);
+  compiler.resolver.resolve(mainElement);
+  Expect.equals(0, compiler.warnings.length);
   Expect.equals(2, compiler.errors.length);
   Expect.equals(MessageKind.CYCLIC_CLASS_HIERARCHY,
                 compiler.errors[0].message.kind);
   Expect.equals(MessageKind.CANNOT_FIND_CONSTRUCTOR.error,
                 compiler.errors[1].message.kind);
 
   compiler = new MockCompiler();
   compiler.parseScript("""abstract class A extends B {}
                           abstract class B extends A {}
                           class C implements A {}
@@ skipping 42 matching lines @@
                           class E extends D {}
                           main() { return new E(); }""");
   mainElement = compiler.mainApp.find(MAIN);
   compiler.resolver.resolve(mainElement);
   Expect.equals(0, compiler.warnings.length);
   Expect.equals(0, compiler.errors.length);
   aElement = compiler.mainApp.find("E");
   supertypes = aElement.allSupertypes;
   Expect.equals(<String>['A<E>', 'D', 'Object'].toString(),
                 asSortedStrings(supertypes).toString());
+
+  compiler = new MockCompiler();
+  compiler.parseScript("""class A<T> {}
+                          class D extends A<int> implements A<double> {}
+                          main() { return new D(); }""");
+  mainElement = compiler.mainApp.find(MAIN);
+  compiler.resolver.resolve(mainElement);
+  Expect.equals(0, compiler.warnings.length);
+  Expect.equals(1, compiler.errors.length);
+  Expect.equals(MessageKind.MULTI_INHERITANCE,
+                compiler.errors[0].message.kind);
 }
 
 testInitializers() {
   String script;
   script = """class A {
                 int foo; int bar;
                 A() : this.foo = 1, bar = 2;
               }""";
   resolveConstructor(script, "A a = new A();", "A", "", 2);
 
@@ skipping 238 matching lines @@
       }""";
   asyncTest(() => compileScript(script2).then((compiler) {
     expect(compiler,
         [MessageKind.CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS],
         [MessageKind.CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_CONSTRUCTOR,
          MessageKind.CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_CONSTRUCTOR,
          MessageKind.CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_FIELD,
          MessageKind.CONST_CONSTRUCTOR_WITH_NONFINAL_FIELDS_FIELD]);
   }));
 }