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Add a stack bitmap to the location summaries for calls that are GC safepoints. The bitmap covers the spill slots. The register allocator collects these bitmaps into a list and then marks live pointer values during register allocation. When emitting code for a call, a heap-allocated stackmap is built. BUG= Review URL: https://chromiumcodereview.appspot.com//10831261 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@10618 260f80e4-7a28-3924-810f-c04153c831b5