[dart2js] Add phase 1 kernel transformer to simplify const conditionals.

Suppose we have code like

if (isDevMode) {
  // do foo
} else {
  // do bar
}

where isDevMode is const (or can be evaluated as const). In particular,
isDevMode can be controlled by compile-time options, like
bool.fromEnvironment.

We currently eliminate the dead branch during SSA, but that means we do
the work of compiling dead code in the previous phases. Instead, we can
recognize that the condition is const (or effectively const) and
eliminate the dead AST subtree directly.

Change-Id: Ia91da5ebc7fa496a1b963308c6e02d572cab936e
Bug: b/254543452
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/270281
Reviewed-by: Sigmund Cherem <sigmund@google.com>