When metadata is not requested, elide unneeded signatures

pkg/dev_compiler/test/codegen_expected/sunflower/sunflower.js

 define(['dart_sdk'], function(dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const html = dart_sdk.html;
   const math = dart_sdk.math;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const sunflower = Object.create(null);
   const circle = Object.create(null);
   const painter = Object.create(null);
@@ skipping 54 matching lines @@
   };
   dart.fn(sunflower.draw, VoidTovoid());
   circle.Circle = class Circle extends core.Object {
     new(x, y, radius) {
       this.x = x;
       this.y = y;
       this.radius = radius;
     }
   };
   dart.setSignature(circle.Circle, {
-    constructors: () => ({new: dart.definiteFunctionType(circle.Circle, [core.num, core.num, core.num])}),
     fields: () => ({
       x: core.num,
       y: core.num,
       radius: core.num
     })
   });
   painter.CirclePainter = class CirclePainter extends core.Object {
     new() {
       this.color = painter.ORANGE;
     }
@@ skipping 13 matching lines @@
     fields: () => ({color: core.String}),
     methods: () => ({draw: dart.definiteFunctionType(dart.void, [html.CanvasRenderingContext2D])})
   });
   sunflower.SunflowerSeed = class SunflowerSeed extends dart.mixin(circle.Circle, painter.CirclePainter) {
     new(x, y, radius, color) {
       if (color === void 0) color = null;
       super.new(x, y, radius);
       if (color != null) this.color = color;
     }
   };
-  dart.setSignature(sunflower.SunflowerSeed, {
-    constructors: () => ({new: dart.definiteFunctionType(sunflower.SunflowerSeed, [core.num, core.num, core.num], [core.String])})
-  });
   painter.ORANGE = "orange";
   painter.RED = "red";
   painter.BLUE = "blue";
   painter.TAU = math.PI * 2;
   painter.querySelector = function(selector) {
     return html.document[dartx.querySelector](selector);
   };
   dart.fn(painter.querySelector, StringToElement());
   dart.defineLazy(painter, {
     get canvas() {
       return html.CanvasElement.as(painter.querySelector("#canvas"));
     }
   });
   dart.defineLazy(painter, {
     get context() {
       return html.CanvasRenderingContext2D.as(painter.canvas[dartx.getContext]('2d'));
     }
   });
   // Exports:
   return {
     sunflower: sunflower,
     circle: circle,
     painter: painter
   };
 });
 
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