Add "class Throwable" and rewrite exception throwing to use JNI.

Change-Id: I79836075337eedfc5923ebff028176615ffd3598
diff --git a/src/jni_compiler_test.cc b/src/jni_compiler_test.cc
index f315d1f..29e8942 100644
--- a/src/jni_compiler_test.cc
+++ b/src/jni_compiler_test.cc
@@ -367,7 +367,7 @@
   EXPECT_EQ(1, gJava_MyClass_foo_calls);
   EXPECT_EQ(0, gExceptionHandler_calls);
   // TODO: create a real exception here
-  Thread::Current()->SetException(reinterpret_cast<Object*>(jobj_));
+  Thread::Current()->SetException(reinterpret_cast<Throwable*>(jobj_));
   env_->CallNonvirtualVoidMethod(jobj_, jklass_, jmethod_);
   EXPECT_EQ(2, gJava_MyClass_foo_calls);
   EXPECT_EQ(1, gExceptionHandler_calls);