Use instanceof when checking Error types in toThrowError

Fixes #819

spec/core/matchers/toThrowErrorSpec.js

@@ -170,7 +170,6 @@ describe("toThrowError", function() {
       result;
 
     CustomError.prototype = new Error();
-    CustomError.prototype.constructor = CustomError;
 
     result = matcher.compare(fn, CustomError);
 
@@ -222,7 +221,6 @@ describe("toThrowError", function() {
       result;
 
     CustomError.prototype = new Error();
-    CustomError.prototype.constructor = CustomError;
 
     result = matcher.compare(fn, CustomError, "foo");
 

src/core/matchers/toThrowError.js

@@ -109,7 +109,7 @@ getJasmineRequireObj().toThrowError = function(j$) {
           return expected === null && errorType === null;
         },
         matches: function(error) {
-          return (errorType === null || error.constructor === errorType) &&
+          return (errorType === null || error instanceof errorType) &&
             (expected === null || messageMatch(error.message));
         }
       };