Avoid printing the full token stream and doing line/column position calculations when extracting th…

runtime/lib/mirrors.cc

Index: runtime/lib/mirrors.cc
diff --git a/runtime/lib/mirrors.cc b/runtime/lib/mirrors.cc
index e98997a2e1476e9f497b0f94fe38a5e1ecab81d7..f8cc5c8b2082a5734a4f6156cb0300c0fa644d38 100644
--- a/runtime/lib/mirrors.cc
+++ b/runtime/lib/mirrors.cc
@@ -2045,6 +2045,12 @@ DEFINE_NATIVE_ENTRY(MethodMirror_source, 1) {
   }
   const Script& script = Script::Handle(func.script());
   const TokenStream& stream = TokenStream::Handle(script.tokens());
+  if (!script.HasSource()) {
+    // When source is not available, avoid printing the whole token stream and
+    // doing expensive position calculations.

[hausner, 2014/02/10 16:49:53]

Maybe mention in the comment why it makes no sense to do the calculation below:
column positions are not available.

Maybe this faster path is good enough even when we have sources? What would we
lose?

+    return stream.GenerateSource(func.token_pos(), func.end_token_pos() + 1);
+  }
+
   const TokenStream::Iterator tkit(stream, func.end_token_pos());
   intptr_t from_line;
   intptr_t from_col;