Improve compilation speed on a pathological case > 2x (7 sec -> 2.7 sec) by improving local lookups.

runtime/vm/scopes.cc

Index: runtime/vm/scopes.cc
===================================================================
--- runtime/vm/scopes.cc	(revision 17040)
+++ runtime/vm/scopes.cc	(working copy)
@@ -294,9 +294,11 @@
 
 
 LocalVariable* LocalScope::LocalLookupVariable(const String& name) const {
+  const intptr_t name_hash = name.StringHash();
   for (intptr_t i = 0; i < variables_.length(); i++) {
     LocalVariable* var = variables_[i];
-    if (var->name().Equals(name) && !var->is_invisible_) {
+    const bool same_hash = var->name().StringHash() == name_hash;
+    if (same_hash && !var->is_invisible_ && var->name().Equals(name)) {
       return var;
     }
   }