fixes #131, use before define from variables to classes

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/constant.dart';
 import 'package:analyzer/src/generated/element.dart';
+import 'package:analyzer/src/generated/error.dart' show ErrorReporter;
+import 'package:analyzer/src/generated/engine.dart' show RecordingErrorListener;
+import 'package:analyzer/src/generated/resolver.dart' show TypeProvider;
 
 /// 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:
 ///
@@ skipping 60 matching lines @@
     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;
     }
   }
 }
+
+class ConstFieldVisitor {
+  final ConstantVisitor _constantVisitor;
+
+  ConstFieldVisitor(TypeProvider types, CompilationUnit unit)
+      : _constantVisitor = new ConstantVisitor.con1(types,
+          new ErrorReporter(new RecordingErrorListener(), unit.element.source));
+
+  // TODO(jmesserly): this is used to determine if the field initialization is
+  // side effect free. We should make the check more general, as things like
+  // list/map literals/regexp are also side effect free and fairly common
+  // to use as field initializers.
+  bool isFieldInitConstant(VariableDeclaration field) =>
+      field.initializer == null || computeConstant(field) != null;
+
+  DartObjectImpl computeConstant(VariableDeclaration field) {
+    // If the constant is already computed by ConstantEvaluator, just return it.
+    VariableElementImpl element = field.element;
+    var result = element.evaluationResult;
+    if (result != null) return result.value;
+
+    // ConstantEvaluator will not compute constants for non-const fields,
+    // so run ConstantVisitor for those to figure out if the initializer is
+    // actually a constant (and therefore side effect free to evaluate).
+    assert(!field.isConst);
+
+    var initializer = field.initializer;
+    if (initializer == null) return null;
+    return initializer.accept(_constantVisitor);
+  }
+}