[fasta] Commandline option to leave environment constants unevaluated.

Also, outdated description of -D option corrected.

Change-Id: Ie1dc011adc3ce2f5293826c96f22700a6528c9a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/96880
Reviewed-by: Peter von der Ahé <ahe@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>

pkg/front_end/lib/src/fasta/fasta_codes_generated.dart

@@ -3329,7 +3329,10 @@ const MessageCode messageFastaUsageLong =
 
   -Dname
   -Dname=value
-    Ignored for now.
+    Define an environment variable in the compile-time environment.
+
+  --no-defines
+    Ignore all -D options and leave environment constants unevaluated.
 
   --
     Stop option parsing, the rest of the command line is assumed to be

pkg/front_end/messages.yaml

@@ -1538,7 +1538,10 @@ FastaUsageLong:
 
       -Dname
       -Dname=value
-        Ignored for now.
+        Define an environment variable in the compile-time environment.
+
+      --no-defines
+        Ignore all -D options and leave environment constants unevaluated.
 
       --
         Stop option parsing, the rest of the command line is assumed to be

pkg/front_end/tool/_fasta/command_line.dart

@@ -251,6 +251,7 @@ const Map<String, dynamic> optionSpecification = const <String, dynamic>{
   "--legacy": "--legacy-mode",
   "--legacy-mode": false,
   "--libraries-json": Uri,
+  "--no-defines": false,
   "--output": Uri,
   "--packages": Uri,
   "--platform": Uri,
@@ -310,6 +311,8 @@ ProcessedOptions analyzeCommandLine(
         "Valid targets are:\n  ${targets.keys.join("\n  ")}");
   }
 
+  final bool noDefines = options["--no-defines"];
+
   final bool enableAsserts = options["--enable-asserts"];
 
   final bool verify = options["--verify"];
@@ -385,7 +388,7 @@ ProcessedOptions analyzeCommandLine(
           ..verify = verify
           ..bytecode = bytecode
           ..experimentalFlags = experimentalFlags
-          ..environmentDefines = parsedArguments.defines,
+          ..environmentDefines = noDefines ? null : parsedArguments.defines,
         inputs: <Uri>[Uri.parse(arguments[0])],
         output: resolveInputUri(arguments[3]));
   } else if (arguments.isEmpty) {
@@ -421,7 +424,7 @@ ProcessedOptions analyzeCommandLine(
     ..verbose = verbose
     ..verify = verify
     ..experimentalFlags = experimentalFlags
-    ..environmentDefines = parsedArguments.defines;
+    ..environmentDefines = noDefines ? null : parsedArguments.defines;
 
   // TODO(ahe): What about chase dependencies?