[ Frontend Server ] Clear error count before compiling expressions

Updated FrontendCompiler.compileExpression() to clear previous errors before
compiling the expression. This matches the behavior of
FrontendCompiler.compileExpressionToJs().

Related issue: flutter/flutter#157922

Change-Id: I7bbc163c3f02758be4a495fd753a4ef6cf03c3ce
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394220
Auto-Submit: Ben Konyi <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: bkonyi

pkg/frontend_server/lib/frontend_server.dart

@@ -1073,6 +1073,7 @@ class FrontendCompiler implements CompilerInterface {
       int offset,
       String? scriptUri,
       bool isStatic) async {
+    errors.clear();
     final String boundaryKey = generateV4UUID();
     _outputStream.writeln('result $boundaryKey');
     Procedure? procedure = await _generator.compileExpression(