[refactor] kick some stuff out of js_codegen.dart

lib/src/codegen/side_effect_analysis.dart

+// Copyright (c) 2015, 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 dev_compiler.src.codegen.side_effect_analysis;
+
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/element.dart';
+
+/// True is the expression can be evaluated multiple times without causing
+/// code execution. This is true for final fields. This can be true for local
+/// variables, if:
+/// * they are not assigned within the [context].
+/// * they are not assigned in a function closure anywhere.
+/// True is the expression can be evaluated multiple times without causing
+/// code execution. This is true for final fields. This can be true for local
+/// variables, if:
+///
+/// * they are not assigned within the [context] scope.
+/// * they are not assigned in a function closure anywhere.
+///
+/// This method is used to avoid creating temporaries in cases where we know
+/// we can safely re-evaluate [node] multiple times in [context]. This lets
+/// us generate prettier code.
+///
+/// This method is conservative: it should never return `true` unless it is
+/// certain the [node] is stateless, because generated code may rely on the
+/// correctness of a `true` value. However it may return `false` for things
+/// that are in fact, stateless.
+bool isStateless(Expression node, [AstNode context]) {
+  if (node is SimpleIdentifier) {
+    var e = node.staticElement;
+    if (e is PropertyAccessorElement) e = e.variable;
+    if (e is VariableElement && !e.isSynthetic) {
+      if (e.isFinal) return true;
+      if (e is LocalVariableElement || e is ParameterElement) {
+        // make sure the local isn't mutated in the context.
+        return !_isPotentiallyMutated(e, context);
+      }
+    }
+  }
+  return false;
+}
+
+/// Returns true if the local variable is potentially mutated within [context].
+/// This accounts for closures that may have been created outside of [context].
+bool _isPotentiallyMutated(VariableElement e, [AstNode context]) {
+  if (e.isPotentiallyMutatedInClosure) return true;
+  if (e.isPotentiallyMutatedInScope) {
+    // Need to visit the context looking for assignment to this local.
+    if (context != null) {
+      var visitor = new _AssignmentFinder(e);
+      context.accept(visitor);
+      return visitor._potentiallyMutated;
+    }
+    return true;
+  }
+  return false;
+}
+
+/// Adapted from VariableResolverVisitor. Finds an assignment to a given
+/// local variable.
+class _AssignmentFinder extends RecursiveAstVisitor {
+  final VariableElement _variable;
+  bool _potentiallyMutated = false;
+
+  _AssignmentFinder(this._variable);
+
+  @override
+  visitSimpleIdentifier(SimpleIdentifier node) {
+    // Ignore if qualified.
+    AstNode parent = node.parent;
+    if (parent is PrefixedIdentifier &&
+        identical(parent.identifier, node)) return;
+    if (parent is PropertyAccess &&
+        identical(parent.propertyName, node)) return;
+    if (parent is MethodInvocation &&
+        identical(parent.methodName, node)) return;
+    if (parent is ConstructorName) return;
+    if (parent is Label) return;
+
+    if (node.inSetterContext() && node.staticElement == _variable) {
+      _potentiallyMutated = true;
+    }
+  }
+}