Use signature inheritance when method types are unchanged

test/codegen/expect/constructors.js

 var constructors = dart.defineLibrary(constructors, {});
 var core = dart.import(core);
 (function(exports, core) {
   'use strict';
   class A extends core.Object {}
-  dart.setSignature(A, {});
   class B extends core.Object {
     B() {
     }
   }
-  dart.setSignature(B, {});
   class C extends core.Object {
     named() {
     }
   }
   dart.defineNamedConstructor(C, 'named');
-  dart.setSignature(C, {});
   class C2 extends C {
     named() {
       super.named();
     }
   }
   dart.defineNamedConstructor(C2, 'named');
-  dart.setSignature(C2, {});
   class D extends core.Object {
     D() {
     }
     named() {
     }
   }
   dart.defineNamedConstructor(D, 'named');
-  dart.setSignature(D, {});
   class E extends core.Object {
     E(name) {
       this.name = name;
     }
   }
-  dart.setSignature(E, {});
   class F extends E {
     F(name) {
       super.E(name);
     }
   }
-  dart.setSignature(F, {});
   class G extends core.Object {
     G(p1) {
       if (p1 === void 0)
         p1 = null;
     }
   }
-  dart.setSignature(G, {});
   class H extends core.Object {
     H(opts) {
       let p1 = opts && 'p1' in opts ? opts.p1 : null;
     }
   }
-  dart.setSignature(H, {});
   class I extends core.Object {
     I() {
       this.name = 'default';
     }
     named(name) {
       this.name = name;
     }
   }
   dart.defineNamedConstructor(I, 'named');
-  dart.setSignature(I, {});
   class J extends core.Object {
     J() {
       this.initialized = true;
       this.nonInitialized = null;
     }
   }
-  dart.setSignature(J, {});
   class K extends core.Object {
     K() {
       this.s = 'a';
     }
     withS(s) {
       this.s = s;
     }
   }
   dart.defineNamedConstructor(K, 'withS');
-  dart.setSignature(K, {});
   class L extends core.Object {
     L(foo) {
       this.foo = foo;
     }
   }
-  dart.setSignature(L, {});
   class M extends L {
     named(x) {
       super.L(dart.notNull(x) + 42);
     }
   }
   dart.defineNamedConstructor(M, 'named');
-  dart.setSignature(M, {});
   class N extends M {
     named(y) {
       super.named(dart.notNull(y) + 100);
     }
   }
   dart.defineNamedConstructor(N, 'named');
-  dart.setSignature(N, {});
   class P extends N {
     P(z) {
       super.named(dart.notNull(z) + 9000);
     }
     foo(x) {
       this.P(dart.notNull(x) + 42);
     }
     bar() {
       this.foo(1);
     }
   }
   dart.defineNamedConstructor(P, 'foo');
   dart.defineNamedConstructor(P, 'bar');
-  dart.setSignature(P, {});
   let Q$ = dart.generic(function(T) {
     class Q extends core.Object {
       Q(y) {
         this.x = dart.as(y, T);
       }
       static foo() {
         return new (Q$())("hello");
       }
       bar() {
         let q = Q$().foo();
@@ skipping 35 matching lines @@
   exports.I = I;
   exports.J = J;
   exports.K = K;
   exports.L = L;
   exports.M = M;
   exports.N = N;
   exports.P = P;
   exports.Q$ = Q$;
   exports.Q = Q;
 })(constructors, core);