[kernel] Fix assert message.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index 3d17400b0046cd784ac662092a11eb7d31bc67db..29a33e122ee900d8d126d359dbbdc386371d7203 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -2657,7 +2657,10 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
     debugEvent("Assert");
     Expression message = popForValueIfNotNull(commaToken);
     Expression condition = popForValue();
-    AssertStatement statement = new AssertStatement(condition, message);
+    AssertStatement statement = new AssertStatement(condition,
+        conditionStartOffset: leftParenthesis.offset + 1,
+        conditionEndOffset: rightParenthesis.offset,
+        message: message);
     switch (kind) {
       case Assert.Statement:
         push(statement);