Use much finer grain dependencies to avoid re-rendering the whole results table all the time.

This makes tests _much_ faster on old browsers where DOM access is slow.
Don't let divs cross isolate boundaries. This fixes IE8.
Comment and reorganize. No functional changes.
rerender the test if new results come in while the test is expanded.
use separate templates instead of isolate.
Merge branch 'test-in-browser-with-finer-deps' into devel
Don't mutate template AST in evaluate.  Fixes #1143
Missing dependency in accounts-ui-unstyled
Lots of missing dependencies, mostly in accounts code.
* tools/packages.js: change an array to an object, since that's how it's used.
Merge branch 'devel' into linker
fix Spark memory leak (#1157)
Merge branch 'devel' into linker
update History.md
Merge branch 'linker' into fix-webapp-routepolicy-dependency
Remove old "weak" name for unordered dependencies.
"meteor rebuild-all" should exit 1 on error
Test slices now never contain exports.

@export is now an error in test slices.

This is preparation for ensuring that any slice with exports has at least
"Package.foo = {}"; without this commit, both the use and test slice would (with
that change) try to overwrite Package.foo.

Remove the attempt to allow @export in 'use strict' CoffeeScript files:
ECMAScript 'use strict' is fundamentally incompatible with our implementation of
exports, and it was probably a bug that this used to work at all.
Implement weak dependencies.

A normal dependency of X on Y consists of a "must-use" constraint ("if X is
loaded, Y must be loaded too") and an "ordering" constraint ("if X and Y are
both loaded, Y must be loaded before X").

The previously-existing support for "unordered" dependencies allows you to
create a dependency with "must-use" but not "ordering".  This commit adds "weak"
dependencies, which have "ordering" but not "must-use".

As an example, the accounts-base package wants to define some Handlebars helpers
like {{currentUser}} if the handlebars package is being used, but it's fine to
use accounts-base without handlebars. So it should declare a weak dependency on
handlebars.

A package can tell if another package has already been loaded by checking to see
if `Packages.foo` exists. (So as a result, even slices which export no variables
get a `Packages.foo = {}` line.) Weak dependencies do not import symbols into
your namespace; you must access their symbols through
`Packages.foo.someExportedSymbol`. You don't get to use plugins from your weak
dependency.
Fix {{currentUser}} and {{loggingIn}} by using weak dependencies.