Fix crash in dartc when given cyclic type variable bounds.

compiler/java/com/google/dart/compiler/type/Types.java

Index: compiler/java/com/google/dart/compiler/type/Types.java
diff --git a/compiler/java/com/google/dart/compiler/type/Types.java b/compiler/java/com/google/dart/compiler/type/Types.java
index d882aee5c96147afe78158230544cb2ac78f60a0..ebe91a742acdcb164af594a14f4762dfef797f29 100644
--- a/compiler/java/com/google/dart/compiler/type/Types.java
+++ b/compiler/java/com/google/dart/compiler/type/Types.java
@@ -18,11 +18,13 @@ import com.google.dart.compiler.resolver.TypeVariableElement;
 import com.google.dart.compiler.resolver.VariableElement;
 
 import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Set;
 
 /**
  * Utility class for types.
@@ -256,6 +258,10 @@ public class Types {
    */
   @VisibleForTesting
   public InterfaceType asInstanceOf(Type t, ClassElement element) {
+    return checkedAsInstanceOf(t, element, new HashSet<TypeVariable>());
+  }
+
+  private InterfaceType checkedAsInstanceOf(Type t, ClassElement element, Set<TypeVariable> variablesReferenced) {
     switch (TypeKind.of(t)) {
       case FUNCTION_ALIAS:
       case INTERFACE: {
@@ -266,13 +272,15 @@ public class Types {
         ClassElement tElement = ti.getElement();
         InterfaceType supertype = tElement.getSupertype();
         if (supertype != null) {
-          InterfaceType result = asInstanceOf(asSupertype(ti, supertype), element);
+          InterfaceType result = checkedAsInstanceOf(asSupertype(ti, supertype), element,
+                                                     variablesReferenced);
           if (result != null) {
             return result;
           }
         }
         for (InterfaceType intrface : tElement.getInterfaces()) {
-          InterfaceType result = asInstanceOf(asSupertype(ti, intrface), element);
+          InterfaceType result = checkedAsInstanceOf(asSupertype(ti, intrface), element,
+                                                     variablesReferenced);
           if (result != null) {
             return result;
           }
@@ -286,7 +294,7 @@ public class Types {
             // e should be the interface Function in the core library. See the
             // documentation comment on FunctionType.
             InterfaceType ti = (InterfaceType) e.getType();
-            return asInstanceOf(ti, element);
+            return checkedAsInstanceOf(ti, element, variablesReferenced);
           default:
             return null;
         }
@@ -294,7 +302,11 @@ public class Types {
       case VARIABLE: {
         TypeVariable v = (TypeVariable) t;
         Type bound = v.getTypeVariableElement().getBound();
-        return asInstanceOf(bound, element);
+        if (variablesReferenced.contains(v)) {
+          return typeProvider.getObjectType();
+        }
+        variablesReferenced.add(v);
+        return checkedAsInstanceOf(bound, element, variablesReferenced);
       }
       default:
         return null;