Implement operator-is for interfaces' supertypes and String's supertypes. Also, improve stack trace…

dart/frog/leg/emitter.dart

Index: dart/frog/leg/emitter.dart
diff --git a/dart/frog/leg/emitter.dart b/dart/frog/leg/emitter.dart
index 48b4ac01f64c419feeaa5cc7bfaa1fd40ef99553..6e27a82fa77e46469ed20f3e711e0445761056ac 100644
--- a/dart/frog/leg/emitter.dart
+++ b/dart/frog/leg/emitter.dart
@@ -408,22 +408,28 @@ if (typeof $dynamicMetadataName == 'undefined') $dynamicMetadataName = [];
       }
     }
     buffer.add('$prototype.${namer.operatorIs(classElement)} = true;\n');
-    for (Type ifc in classElement.interfaces) {
-      buffer.add('$prototype.${namer.operatorIs(ifc.element)} = true;\n');
-    }
+    generateInterfacesIsTests(buffer, prototype, classElement);
     if (superclass === null) {
       // JS "toString" wrapper. This gives better exceptions.
       buffer.add('$prototype.toString = function() {\n');
-      buffer.add('  try {');
+      buffer.add('  try {\n');
       buffer.add('    return ${namer.CURRENT_ISOLATE}');
-      buffer.add('.builtin\$toString\$0\$1(this);');
-      buffer.add('  } catch (ex) {');
-      buffer.add('     return "uncaught exception in toString";');
-      buffer.add('  }');
+      buffer.add('.builtin\$toString\$0\$1(this);\n');
+      buffer.add('  } catch (ex) {\n');
+      buffer.add('     return "uncaught exception in toString";\n');
+      buffer.add('  }\n');
       buffer.add('};\n');
     }
   }
 
+  void generateInterfacesIsTests(StringBuffer buffer, String prototype,
+                                 ClassElement cls) {
+    for (Type ifc in cls.interfaces) {
+      buffer.add('$prototype.${namer.operatorIs(ifc.element)} = true;\n');
+      generateInterfacesIsTests(buffer, prototype, ifc.element);
+    }
+  }
+
   void emitClasses(StringBuffer buffer) {
     Set seenClasses = new Set<ClassElement>();
     for (ClassElement element in compiler.universe.instantiatedClasses) {