[dartdevc] preliminary nnbd support

For analyzer backend only:
* Implements `x!`
* Uses non-null type info where possible to elide null checks.
* Does not yet reify.

No hurry on this - starting to look at what's there.  Some notes:
* NNBD doesn't seem to be exposed directly on DartType yet.
* x!.y doesn't seem to parse - need to write (x!).y.
* Can't run existing tests yet - `package:expect`, etc., have NNBD errors.

Change-Id: I58c24c950d00f3a40d789d4a7adf049786977c50
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/103040
Commit-Queue: Vijay Menon <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: vsmenon

pkg/dev_compiler/lib/src/analyzer/code_generator.dart

@@ -5038,6 +5038,11 @@ class CodeGenerator extends Object
     var op = node.operator;
     var expr = node.operand;
 
+    if (op.type == TokenType.BANG) {
+      // If the expression is non-nullable already, this is a no-op.
+      return isNullable(expr) ? notNull(expr) : _visitExpression(expr);
+    }
+
     var dispatchType = getStaticType(expr);
     if (jsTypeRep.unaryOperationIsPrimitive(dispatchType)) {
       if (!isNullable(expr)) {