Restructure the Kernel string table.

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 28 matching lines @@
   Byte byte3(xxxxxxxx);
   Byte byte4(xxxxxxxx); // least significant byte
 }
 
 type MagicWord = big endian 32-bit unsigned integer
 
 type List<T> {
   UInt length;
   T[length] items;
 }
+```
 
-type String {
-  List<Byte> utf8Bytes;
-}
+A string table consists of an array of end offsets and a payload array of
+strings encoded as UTF-8.  The array of end offsets maps a string index to the
+offset of the _next_ string in the table or the offset of the end of the array
+for the last string.  These offsets are relative to the string payload array.
+Thus, string number 0 consists of the UTF-8 encoded string stretching from
+offset 0 (inclusive) to endOffset[0] (exclusive); and string number N for N > 0
+consists of the UTF-8 encoded string stretching from offset endOffset[N-1]
+(inclusive) to endOffset[N] (exclusive).
 
+``` scala
 type StringTable {
-  List<String> strings;
+  List<UInt> endOffsets;
+  Byte[endOffsets.last] utf8Bytes;
 }
 
 type StringReference {
-  UInt index; // Index into the StringTable strings.
+  UInt index; // Index into the Program's .strings.
 }
 
 type Source {
-  String source;
+  List<Byte> utf8Bytes;
   // Line starts are delta-encoded (they are encoded as line lengths).  The list
   // [0, 10, 25, 32, 42] is encoded as [0, 10, 15, 7, 10].
   List<Uint> lineStarts;
 }
 
 type UriSource {
-  List<String> uris;
-  Source[uris.length] source;
+  StringTable uris;
+  Source[uris.endOffsets.length] source;
 }
 
 type UriReference {
   UInt index; // Index into the UriSource uris.
 }
 
 type FileOffset {
   // Encoded as offset + 1 to accommodate -1 indicating no offset.
   UInt fileOffset;
 }
@@ skipping 865 matching lines @@
   Byte tag = 88;
 }
 
 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.
 }
 
 ```