Pull JS assignments into var initializer.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
@@ skipping 41 matching lines @@
 
   CodeGenerator(this.glue, this.registry);
 
   /// Generates JavaScript code for the body of [function].
   js.Fun buildFunction(tree_ir.FunctionDefinition function) {
     currentFunction = function.element;
     visitStatement(function.body);
 
     List<js.Parameter> parameters = new List<js.Parameter>();
     Set<tree_ir.Variable> parameterSet = new Set<tree_ir.Variable>();
+    Set<String> declaredVariables = new Set<String>();
 
     for (tree_ir.Variable parameter in function.parameters) {
       String name = getVariableName(parameter);
       parameters.add(new js.Parameter(name));
       parameterSet.add(parameter);
+      declaredVariables.add(name);
     }
 
     List<js.VariableInitialization> jsVariables = <js.VariableInitialization>[];
 
+    // Declare variables with an initializer. Pull statements into the
+    // initializer until we find a statement that cannot be pulled in.
+    int accumulatorIndex = 0;
+    while (accumulatorIndex < accumulator.length) {
+      js.Node node = accumulator[accumulatorIndex];
+
+      // Check that node is an assignment to a local variable.
+      if (node is! js.ExpressionStatement) break;
+      js.ExpressionStatement stmt = node;
+      if (stmt.expression is! js.Assignment) break;
+      js.Assignment assign = stmt.expression;
+      if (assign.leftHandSide is! js.VariableUse) break;
+      if (assign.op != null) break; // Compound assignment.
+      js.VariableUse use = assign.leftHandSide;
+
+      // We cannot declare a variable more than once.
+      if (!declaredVariables.add(use.name)) break;
+
+      js.VariableInitialization jsVariable = new js.VariableInitialization(
+        new js.VariableDeclaration(use.name),
+        assign.value);
+      jsVariables.add(jsVariable);
+
+      ++accumulatorIndex;
+    }
+
+    // Discard the statements that were pulled in the initializer.
+    if (accumulatorIndex > 0) {
+      accumulator = accumulator.sublist(accumulatorIndex);
+    }
+
+    // Declare remaining variables.
     for (tree_ir.Variable variable in variableNames.keys) {
-      if (parameterSet.contains(variable)) continue;
       String name = getVariableName(variable);
+      if (declaredVariables.contains(name)) continue;
       js.VariableInitialization jsVariable = new js.VariableInitialization(
         new js.VariableDeclaration(name),
         null);
       jsVariables.add(jsVariable);
     }
 
     if (jsVariables.length > 0) {
       // Would be nice to avoid inserting at the beginning of list.
       accumulator.insert(0, new js.ExpressionStatement(
           new js.VariableDeclarationList(jsVariables)));
@@ skipping 16 matching lines @@
     // Functions are not nested in the JS backend.
     assert(variable.host == currentFunction);
 
     // Get the name if we already have one.
     String name = variableNames[variable];
     if (name != null) {
       return name;
     }
 
     // Synthesize a variable name that isn't used elsewhere.
-    // The [usedVariableNames] set is shared between nested emitters,
-    // so this also prevents clash with variables in an enclosing/inner scope.
-    // The renaming phase after codegen will further prefix local variables
-    // so they cannot clash with top-level variables or fields.
     String prefix = variable.element == null ? 'v' : variable.element.name;
     int counter = 0;
     name = glue.safeVariableName(variable.element == null
         ? '$prefix$counter'
         : variable.element.name);
     while (!usedVariableNames.add(name)) {
       ++counter;
       name = '$prefix$counter';
     }
     variableNames[variable] = name;
@@ skipping 469 matching lines @@
 
   @override
   visitFunctionDeclaration(tree_ir.FunctionDeclaration node) {
     return errorUnsupportedNode(node);
   }
 
   errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }
 }