perf(mergeMapTo): remove tryCatch/errorObject (~2x improvement)

Before:
                                      |     RxJS 4.0.7 | RxJS 5.0.0-beta.1 | factor | % improved
-------------------------------------------------------------------------------------------------
mergemapto-resultselector - immediate |   811 (±1.50%) |    5,376 (±1.10%) |  6.63x |     562.7%
        mergemapto-scalar - immediate | 6,110 (±0.45%) |  113,487 (±0.91%) | 18.58x |   1,757.5%
               mergemapto - immediate | 1,439 (±0.95%) |   17,017 (±0.29%) | 11.83x |   1,082.8%
            mergemapto-resultselector |   369 (±0.43%) |    1,957 (±0.45%) |  5.30x |     429.7%
                    mergemapto-scalar | 3,740 (±0.45%) |   66,278 (±1.18%) | 17.72x |   1,672.3%
                           mergemapto |   490 (±0.39%) |    3,685 (±0.44%) |  7.52x |     651.7%

After:
                                      |     RxJS 4.0.7 | RxJS 5.0.0-beta.1 | factor | % improved
-------------------------------------------------------------------------------------------------
mergemapto-resultselector - immediate |   810 (±1.70%) |   11,428 (±3.41%) | 14.11x |   1,310.6%
        mergemapto-scalar - immediate | 6,401 (±2.48%) |  117,259 (±2.09%) | 18.32x |   1,731.9%
               mergemapto - immediate | 1,452 (±0.44%) |   17,406 (±0.35%) | 11.99x |   1,098.9%
            mergemapto-resultselector |   383 (±1.24%) |    3,288 (±0.42%) |  8.58x |     757.7%
                    mergemapto-scalar | 3,837 (±0.57%) |   62,294 (±3.05%) | 16.23x |   1,523.3%
                           mergemapto |   525 (±2.37%) |    3,795 (±0.79%) |  7.23x |     622.6%

src/operator/mergeMapTo.ts

@@ -2,8 +2,6 @@ import {Observable} from '../Observable';
 import {Operator} from '../Operator';
 import {PartialObserver} from '../Observer';
 import {Subscriber} from '../Subscriber';
-import {tryCatch} from '../util/tryCatch';
-import {errorObject} from '../util/errorObject';
 import {Subscription} from '../Subscription';
 import {OuterSubscriber} from '../OuterSubscriber';
 import {InnerSubscriber} from '../InnerSubscriber';
@@ -73,17 +71,26 @@ export class MergeMapToSubscriber<T, R, R2> extends OuterSubscriber<T, R> {
              innerSub: InnerSubscriber<T, R>): void {
     const { resultSelector, destination } = this;
     if (resultSelector) {
-      const result = tryCatch(resultSelector)(outerValue, innerValue, outerIndex, innerIndex);
-      if (result === errorObject) {
-        destination.error(errorObject.e);
-      } else {
-        destination.next(result);
-      }
+      this.trySelectResult(outerValue, innerValue, outerIndex, innerIndex);
     } else {
       destination.next(innerValue);
     }
   }
 
+  private trySelectResult(outerValue: T, innerValue: R,
+                          outerIndex: number, innerIndex: number): void {
+    const { resultSelector, destination } = this;
+    let result: R2;
+    try {
+      result = resultSelector(outerValue, innerValue, outerIndex, innerIndex);
+    } catch (err) {
+      destination.error(err);
+      return;
+    }
+
+    destination.next(result);
+  }
+
   notifyError(err: any): void {
     this.destination.error(err);
   }