fixes #52, fields shadowing getters/setters or other fields

test/codegen/expect/constructors.js

 var constructors;
 (function(exports) {
   'use strict';
   class A extends core.Object {}
   class B extends core.Object {
     B() {
     }
   }
   class C extends core.Object {
     named() {
     }
   }
   dart.defineNamedConstructor(C, 'named');
   class C2 extends C {
     named() {
       super.named();
     }
   }
   dart.defineNamedConstructor(C2, 'named');
   class D extends core.Object {
     D() {
     }
     named() {
     }
   }
   dart.defineNamedConstructor(D, 'named');
   class E extends core.Object {
     E(name) {
-      this.name = name;
+      dart.initField(E, this, 'name', name);
     }
   }
   class F extends E {
     F(name) {
       super.E(name);
     }
   }
   class G extends core.Object {
     G(p1) {
       if (p1 === void 0)
         p1 = null;
     }
   }
   class H extends core.Object {
     H(opts) {
       let p1 = opts && 'p1' in opts ? opts.p1 : null;
     }
   }
   class I extends core.Object {
     I() {
-      this.name = 'default';
+      dart.initField(I, this, 'name', 'default');
     }
     named(name) {
-      this.name = name;
+      dart.initField(I, this, 'name', name);
     }
   }
   dart.defineNamedConstructor(I, 'named');
   class J extends core.Object {
     J() {
-      this.initialized = true;
-      this.nonInitialized = null;
+      dart.initField(J, this, 'initialized', true);
+      dart.initField(J, this, 'nonInitialized', null);
     }
   }
   class K extends core.Object {
     K() {
-      this.s = 'a';
+      dart.initField(K, this, 's', 'a');
     }
     withS(s) {
-      this.s = s;
+      dart.initField(K, this, 's', s);
     }
   }
   dart.defineNamedConstructor(K, 'withS');
   class L extends core.Object {
     L(foo) {
-      this.foo = foo;
+      dart.initField(L, this, 'foo', foo);
     }
   }
   class M extends L {
     named(x) {
       super.L(dart.notNull(x) + 42);
     }
   }
   dart.defineNamedConstructor(M, 'named');
   class N extends M {
     named(y) {
@@ skipping 25 matching lines @@
   exports.G = G;
   exports.H = H;
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
 })(constructors || (constructors = {}));