[kernel] Fix assert message.

pkg/kernel/lib/binary/ast_from_binary.dart

Index: pkg/kernel/lib/binary/ast_from_binary.dart
diff --git a/pkg/kernel/lib/binary/ast_from_binary.dart b/pkg/kernel/lib/binary/ast_from_binary.dart
index b730ddebe8f39e0d6edf81f0d4abd0e3a792aea7..4047e49ccfa8dfb70c690d81556c766c158f6162 100644
--- a/pkg/kernel/lib/binary/ast_from_binary.dart
+++ b/pkg/kernel/lib/binary/ast_from_binary.dart
@@ -935,7 +935,12 @@ class BinaryBuilder {
       case Tag.EmptyStatement:
         return new EmptyStatement();
       case Tag.AssertStatement:
-        return new AssertStatement(readExpression(), readExpressionOption());
+        Expression condition = readExpression();
+        int conditionStart = readOffset();
+        int conditionEnd = readOffset();
+        return new AssertStatement(condition, readExpressionOption())
+          ..conditionStartOffset = conditionStart
+          ..conditionEndOffset = conditionEnd;
       case Tag.LabeledStatement:
         var label = new LabeledStatement(null);
         labelStack.add(label);