[BUGFIX beta][FEAT] Adds a second dist build which targets IE

[pzuraq]

This PR adds a second dist to the build which targets IE. This allows
the primary bundles to drop ES5 compilation from babel, and allows us to
choose which bundles we want to use based on the consuming application's
targets.

In the long run, we want to actually enable consuming apps to build the
Ember source code themselves. This is a temporary in-between measure to
address a bug in native classes, in which users who do not have IE as
a build target cannot .extend from native classes due to conflicts
in transpilation vs actual native class syntax.

[rwjblue]

this shouldn't be checked in (maybe we should add it to .gitignore?)

[rwjblue]

Oh, also can you update the commit message to start with [BUGFIX beta]?

[pzuraq]

ALrighty, restructured to a better flow:

1. Gather all files
2. Transpile to ES5
3. Debug macros + strip and prep for production
4. Gather files into bundles
5. Transform AMD and concat files

This flow ended up making the most sense to me given that we must do
the AMD transform last. It keeps most major operations at a per-file
level, and saves the file-divying and concatting for last.

Anecdotally, the restructure does seem to be a bit slower, and there are some inconsistencies in the final builds. Specifically, ember.prod.js seems to contain more comments and have less code removed by the minifier. ember.min.js seems to be mostly unaffected:

# before
 - dist/ember-template-compiler.js: 404.07 KB (80.96 KB gzipped)
 - dist/ember-testing.js: 78.35 KB (17.96 KB gzipped)
 - dist/ember-tests.js: 3.15 MB (417.24 KB gzipped)
 - dist/ember-tests.prod.js: 1.06 MB (141.87 KB gzipped)
 - dist/ember.debug.js: 1.74 MB (357.91 KB gzipped)
 - dist/ember.min.js: 469.15 KB (120.22 KB gzipped)
 - dist/ember.prod.js: 1.6 MB (329.29 KB gzipped)

# after
 - dist/ie/ember-template-compiler.js: 408.98 KB (81.99 KB gzipped)
 - dist/ie/ember-testing.js: 78.93 KB (17.95 KB gzipped)
 - dist/ie/ember-tests.js: 3.17 MB (420.78 KB gzipped)
 - dist/ie/ember-tests.prod.js: 1.07 MB (143.26 KB gzipped)
 - dist/ie/ember.debug.js: 1.91 MB (399.59 KB gzipped)
 - dist/ie/ember.min.js: 469.38 KB (120.36 KB gzipped)
 - dist/ie/ember.prod.js: 1.77 MB (371.62 KB gzipped)

[rwjblue]

Hmm, I would not have expected the sizes to differ so much, would you mind attaching a .tar.gz of the dist/ (before and after) so I can poke through it?

[pzuraq]

Sure thing!

dist-dual-build.tar.gz
dist-master.tar.gz

[pzuraq]

I've been diffing the output bundles and while I haven't checked every single difference, a lot of them seem to come to a few categories:

1. Comments are not being removed from the final output, when they previously were. Comments are still removed from the final min build, so this isn't really a concern for that build and in fact they probably should not have been removed in the other builds.

2. Dead code elimination isn't quite working in the ember.prod.js build. Specifically, it appears to do a single pass and remove tautologies, but then does not remove functions which were only used in those tautological branches. This again does not affect the min build (verified by reading the actual function code).

3. Very small differences in the way transpilation/minification occurs. For instance, in one build an extra pair of enclosing parens may be emitted around an expression. In another, a variable which was inlined was no longer inlined.

Surprisingly, removing the minify-dead-code-elimination plugin actually reduces the final build size to below the original final build, so it seems like the plugin may have been interfering with the minification process more than helping.

# before
 - dist/ie/ember-template-compiler.js: 408.98 KB (81.99 KB gzipped)
 - dist/ie/ember-testing.js: 78.93 KB (17.95 KB gzipped)
 - dist/ie/ember-tests.js: 3.17 MB (420.78 KB gzipped)
 - dist/ie/ember-tests.prod.js: 1.07 MB (143.26 KB gzipped)
 - dist/ie/ember.debug.js: 1.91 MB (399.59 KB gzipped)
 - dist/ie/ember.min.js: 469.38 KB (120.36 KB gzipped)
 - dist/ie/ember.prod.js: 1.77 MB (371.62 KB gzipped)

# after
 - dist/ie/ember-template-compiler.js: 417.36 KB (83 KB gzipped)
 - dist/ie/ember-testing.js: 78.93 KB (17.95 KB gzipped)
 - dist/ie/ember-tests.js: 3.17 MB (420.78 KB gzipped)
 - dist/ie/ember-tests.prod.js: 1.11 MB (154.61 KB gzipped)
 - dist/ie/ember.debug.js: 1.91 MB (399.59 KB gzipped)
 - dist/ie/ember.min.js: 470.61 KB (120.07 KB gzipped)
 - dist/ie/ember.prod.js: 1.84 MB (386.31 KB gzipped)

# non-ie build
 - dist/ember-template-compiler.js: 385.2 KB (79.08 KB gzipped)
 - dist/ember-testing.js: 76.5 KB (17.55 KB gzipped)
 - dist/ember-tests.js: 2.54 MB (340.77 KB gzipped)
 - dist/ember-tests.prod.js: 954.85 KB (130.86 KB gzipped)
 - dist/ember.debug.js: 1.75 MB (378.37 KB gzipped)
 - dist/ember.min.js: 407.55 KB (113.57 KB gzipped)
 - dist/ember.prod.js: 1.71 MB (367.31 KB gzipped)