ReadableStream.cancel's parameter should be optional.

Source/modules/fetch/Body.cpp

Index: Source/modules/fetch/Body.cpp
diff --git a/Source/modules/fetch/Body.cpp b/Source/modules/fetch/Body.cpp
index 3196a3c7d027faae92d3c38ebb3e24f4c83980d5..045b6a224edccda5784910e92e520ca7cc1d9912 100644
--- a/Source/modules/fetch/Body.cpp
+++ b/Source/modules/fetch/Body.cpp
@@ -141,7 +141,9 @@ private:
     void pullSource() override { }
     ScriptPromise cancelSource(ScriptState* scriptState, ScriptValue reason) override
     {
-        return ScriptPromise();
+        close();
+        m_bodyStreamBuffer->unregisterObserver();
+        return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));

[tyoshino (SeeGerritForStatus), 2015/03/23 03:57:21]

which call terminates the fetch?

[yhirano, 2015/03/23 05:47:32]

You're right, and I found implementing the cancellation operation is a bit hard.
So I would fix the IDL related issue in this CL and fix the fetch termination
after that.

     }
 
     // BodyStreamBuffer::Observer functions.