Use destructuring assignments for named parameters (#180)

lib/src/js/printer.dart

 // Copyright (c) 2012, 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.
 
 part of js_ast;
 
 
 class JavaScriptPrintingOptions {
   final bool shouldCompressOutput;
   final bool minifyLocalVariables;
@@ skipping 577 matching lines @@
   }
 
   visitVariableDeclarationList(VariableDeclarationList list) {
     outClosureAnnotation(list);
     out(list.keyword);
     out(" ");
     visitCommaSeparated(list.declarations, ASSIGNMENT,
                         newInForInit: inForInit, newAtStatementBegin: false);
   }
 
+  visitArrayDestructuring(ArrayDestructuring node) {
+    out("[");
+    visitCommaSeparated(node.variables, EXPRESSION,
+                        newInForInit: false, newAtStatementBegin: false);
+    out("]");
+  }
+  visitObjectDestructuring(ObjectDestructuring node) {
+    out("{");
+    visitCommaSeparated(node.variables, EXPRESSION,
+                        newInForInit: false, newAtStatementBegin: false);
+    out("}");
+  }
+
+  visitDestructuredVariable(DestructuredVariable node) {
+    var hasName = node.name != null;
+    if (hasName) visit(node.name);
+    if (node.structure != null) {
+      if (hasName) {
+        out(":");
+        spaceOut();
+      }
+      visit(node.structure);
+    }
+    if (node.defaultValue != null) {
+      spaceOut();
+      out("=");
+      spaceOut();
+      visitNestedExpression(node.defaultValue, EXPRESSION,
+                            newInForInit: false, newAtStatementBegin: false);
+    }
+  }
+
   visitAssignment(Assignment assignment) {
     visitNestedExpression(assignment.leftHandSide, LEFT_HAND_SIDE,
                           newInForInit: inForInit,
                           newAtStatementBegin: atStatementBegin);
     if (assignment.value != null) {
       spaceOut();
       String op = assignment.op;
       if (op != null) out(op);
       out("=");
       spaceOut();
@@ skipping 857 matching lines @@
     return newName;
   }
 }
 
 /// Like [BaseVisitor], but calls [declare] for [Identifier] declarations, and
 /// [visitIdentifier] otherwise.
 abstract class VariableDeclarationVisitor<T> extends BaseVisitor<T> {
   declare(Identifier node);
 
   visitFunctionExpression(FunctionExpression node) {
-    for (var p in node.params) {
-      declare(p is RestParameter ? p.parameter : p);
-    }
+    node.params.forEach(_scanDeclarator);
     node.body.accept(this);
   }
 
+  _scanDeclarator(Declarator d) {
+    if (d is Identifier) declare(d);
+    if (d is RestParameter) _scanDeclarator(d.parameter);
+    else if (d is Destructuring) {
+      for (var v in d.variables) {
+        if (v.name != null) declare(v.name);
+        if (v.structure != null) _scanDeclarator(v.structure);
+      }
+    }
+  }
+
   visitVariableInitialization(VariableInitialization node) {
-    declare(node.declaration);
+    _scanDeclarator(node.declaration);
     if (node.value != null) node.value.accept(this);
   }
 
   visitCatch(Catch node) {
     declare(node.declaration);
     node.body.accept(this);
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     declare(node.name);
     node.function.accept(this);
   }
 
   visitNamedFunction(NamedFunction node) {
     declare(node.name);
     node.function.accept(this);
   }
 
   visitClassExpression(ClassExpression node) {
     declare(node.name);
     if (node.heritage != null) node.heritage.accept(this);
     for (Method element in node.methods) element.accept(this);
   }
 }