fix(build): reorder bundling step

[robwormald]

Previously, each bundle gulp task would kick off 3 bundle processes in parallel, and the tasks themselves are kicked off in parallel. This leads to 9+ bundle / source-mapping / minification tasks running in parallel, and causes CI to be quite flaky, exhibiting errors like:

Security context: 0x21ef97b37399 <JS Object>
    2: _serializeMappings(aka SourceMapGenerator_serializeMappings) [/home/travis/build/angular/angular/node_modules/systemjs-builder/node_modules/source-map/lib/source-map/source-map-generator.js:~289] [pc=0x244e65cef4ed] (this=0x2c83a9f9e379 <a SourceMapGenerator with map 0x2b2aab242ed1>)
    3: toJSON(aka SourceMapGenerator_toJSON) [/home/travis/build/angular/angular/node_modules/systemjs-bu...
FATAL ERROR: CALL_AND_RETRY_LAST Allocation failed - process out of memory

This reorders each task slightly so the main bundle step completes before the aux modules get bundled.

[mary-poppins]

Merging PR #5208 on behalf of @jeffbcross to branch presubmit-jeffbcross-pr-5208.

[mary-poppins]

Merging PR #5208 on behalf of @jeffbcross to branch presubmit-jeffbcross-pr-5208.

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