[kernel] Fix assert message.

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 733 matching lines @@
   List<Expression> expressions;
 }
 
 type EmptyStatement extends Statement {
   Byte tag = 63;
 }
 
 type AssertStatement extends Statement {
   Byte tag = 64;
   Expression condition;
+  FileOffset conditionStartOffset;
+  FileOffset conditionEndOffset;
   Option<Expression> message;
 }
 
 type LabeledStatement extends Statement {
   Byte tag = 65;
   Statement body;
 }
 
 type BreakStatement extends Statement {
   Byte tag = 66;
@@ skipping 232 matching lines @@
   Option<DartType> bound;
 }
 
 type TypeParameter {
   // Note: there is no tag on TypeParameter
   StringReference name; // Cosmetic, may be empty, not unique.
   DartType bound; // 'dynamic' if no explicit bound was given.
 }
 
 ```