Enable consts from environment in DDK for flutter_web (flutter/engine#10069)

Fixes: https://github.com/dart-lang/sdk/issues/37605

We were seeing unevaluated constants during the build for flutter_web. Passing an empty map is handled differently than null so the constant evaluation will kick in.

engine/src/flutter/web_sdk/flutter_kernel_sdk.dart

@@ -63,7 +63,8 @@ Future main(List<String> args) async {
     ..packagesFileUri = Uri.base.resolveUri(Uri.file(packagesPath))
     ..librariesSpecificationUri = Uri.base.resolveUri(Uri.file(librarySpecPath))
     ..target = target
-    ..onDiagnostic = onDiagnostic;
+    ..onDiagnostic = onDiagnostic
+    ..environmentDefines = {};
 
   var inputs = target.extraRequiredLibraries.map(Uri.parse).toList();
   var component = await kernelForComponent(inputs, options);