Make goog.Promise compatible with the strictCheckTypes compiler check.
Make goog.Promise compatible with the strictCheckTypes compiler check.

Before this CL the compilation failed with the following errors:

javascript/closure/promise/promise.js:1207: ERROR - Property stack never defined on *
  if (goog.Promise.LONG_STACK_TRACES && err && goog.isString(err.stack) &&
                                                                 ^^^^^

javascript/closure/promise/promise.js:1222: ERROR - Property stack never defined on *
    err.stack += '\n\n' + longTrace.join('\n');
        ^^^^^

RELNOTES: n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=210579808

lucidsoftware/closure-library,lucidsoftware/closure-library,lucidsoftware/closure-library,google/closure-library,teppeis/closure-library,google/closure-library,google/closure-library,teppeis/closure-library,google/closure-library,lucidsoftware/closure-library,teppeis/closure-library,google/closure-library,teppeis/closure-library