fix(tsc-wrapped): use tsickle's new source map composition feature (#14150)

Tsickle transforms typescript code, which can change the location of code.
This can cause issues such as runtime stack traces reporting that errors
were raised on the incorrect line in the orginal source.  This change replaces
the DecoratorDownlevelCompilerHost and the TsickleCompilerHost with tsickle's
TsickleCompilerHost, which automatically composes tsickle's source maps with
typescript's source maps, so that line numbers are correctly reported at
runtime.

PR Close #14150

package.json

@@ -86,7 +86,7 @@
     "source-map-support": "^0.4.2",
     "systemjs": "0.18.10",
     "ts-api-guardian": "0.1.4",
-    "tsickle": "^0.2.4",
+    "tsickle": "^0.2.6",
     "tslint": "^4.1.1",
     "tslint-eslint-rules": "^3.1.0",
     "typescript": "^2.0.2",