tree-ir: Do not merge captured parameters with other variables.

pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
index 32a8cffb065d6e313599dec255ff5bc4a3d16b9f..fb69832b0009e7044a3436479313c5e32c27acfa 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
@@ -421,6 +421,7 @@ Map<Variable, Variable> _computeRegisterAllocation(List<Block> blocks,
   // cannot be merged with each other. Ensure that they are not substituted.
   // Other variables can still be substituted by a parameter.
   for (Variable parameter in parameters) {
+    if (parameter.isCaptured) continue;

[asgerf, 2015/04/15 10:43:39]

This is a bug that snuck in with the last change.

     subst[parameter] = parameter;
     registers[group(parameter)] = <Variable>[parameter];
   }