perf(bracket): Optimize calling methods on objects.

Optimize the closures used for calling methods on objects by getting rid
of an intermediate "field access" that loads the function from the object.
This allows us to use InstanceMirror.invoke more directly.

Performance
===========
The performance implications of this change look good. On the native Dart VM,
calling methods through the reflective path becomes 4.5x faster. When compiled
to JavaScript, the difference is even bigger and it is 6.7x faster.

Dart VM:
  new: o.f() => g: 18,885,385.384 ops/sec  r: 2,252,149.057 ops/sec   h: 2,233,514.96 ops/sec   i: 32,662,458.941 ops/sec
  old: o.f() => g: 18,118,349.692 ops/sec  r:   496,119.238 ops/sec   h: 6,234,893.68 ops/sec   i: 31,472,943.499 ops/sec

Dart2js:
  new: o.f() => g:  2,618,328.337 ops/sec  r: 1,234,723.632 ops/sec   h: 1,234,745.10 ops/sec   i:  7,507,441.131 ops/sec
  old: o.f() => g:  2,427,133.253 ops/sec  r:   184,727.903 ops/sec   h:   500,226.70 ops/sec   i:  7,461,638.467 ops/sec

bin/parser_generator_for_spec.dart

@@ -27,6 +27,7 @@ main(arguments) {
       "a.b.c",
       "x.b.c",
       "e1.b",
+      "o.f()",
       "1", "-1", "+1",
       "!true",
       "3*4/2%5", "3+6-2",

lib/core/parser/backend.dart

@@ -4,7 +4,7 @@ class BoundExpression {
   var _context;
   Expression expression;
 
-  BoundExpression(this._context, this.expression);
+  BoundExpression(this._context, Expression this.expression);
 
   call([locals]) => expression.eval(_context, locals);
   assign(value, [locals]) => expression.assign(_context, value, locals);
@@ -17,7 +17,15 @@ class Expression implements ParserAST {
   String exp;
   List parts;
 
-  Expression(this.eval, [this.assign]);
+  // Expressions that represent field accesses have a couple of
+  // extra fields. We use that to generate an optimized closure
+  // for calling fields of objects without having to load the
+  // field separately.
+  Expression fieldHolder;
+  String fieldName;
+  bool get isFieldAccess => fieldHolder != null;
+
+  Expression(ParsedGetter this.eval, [ParsedSetter this.assign]);
 
   bind(context) => new BoundExpression(context, this);
 
@@ -93,7 +101,7 @@ class ParserBackend {
   GetterSetter _getterSetter;
   FilterMap _filters;
 
-  ParserBackend(this._getterSetter, this._filters);
+  ParserBackend(GetterSetter this._getterSetter, FilterMap this._filters);
 
   static Expression ZERO = new Expression((_, [_x]) => 0);
 
@@ -314,7 +322,9 @@ class FilterExpression extends Expression {
   final Expression leftHandSide;
   final List<Expression> parameters;
 
-  FilterExpression(this.filterFn, this.leftHandSide, this.parameters): super(null);
+  FilterExpression(Function this.filterFn,
+                   Expression this.leftHandSide,
+                   List<Expression> this.parameters): super(null);
 
   get eval => _eval;
 

perf/parser_perf.dart

@@ -38,6 +38,7 @@ main() {
 
   scope['a'] = new ATest();
   scope['e1'] = new EqualsThrows();
+  scope['o'] = new OTest();
 
   compare(expr, idealFn) {
     var nf = new NumberFormat.decimalPattern();
@@ -63,6 +64,7 @@ main() {
 
   compare('a.b.c', (scope) => scope['a'].b.c);
   compare('e1.b', (scope) => scope['e1'].b);
+  compare('o.f()', (scope) => scope['o'].f());
   compare('null', (scope) => null);
   compare('x.b.c', (s, [l]) {
     if (l != null && l.containsKey('x')) s = l['x'];
@@ -83,6 +85,10 @@ class BTest {
   var c = 6;
 }
 
+class OTest {
+  f() => 42;
+}
+
 class EqualsThrows {
   var b = 3;
   operator ==(x) {