Upgrade to coverage 3.7.1 and have it auto-build itself on first use.

third_party/coverage-3.7.1/coverage/backward.py

Index: third_party/coverage-3.7.1/coverage/backward.py
diff --git a/third_party/coverage-3.6/coverage/backward.py b/third_party/coverage-3.7.1/coverage/backward.py
similarity index 77%
rename from third_party/coverage-3.6/coverage/backward.py
rename to third_party/coverage-3.7.1/coverage/backward.py
index 2c015af2ba1bcdb9897a78f291929c21397398fd..7d2685459782c85c59ff7b78cf8d02adf4b2ca89 100644
--- a/third_party/coverage-3.6/coverage/backward.py
+++ b/third_party/coverage-3.7.1/coverage/backward.py
@@ -24,6 +24,15 @@ except NameError:
         lst.sort()
         return lst
 
+# Python 2.3 doesn't have `reversed`.
+try:
+    reversed = reversed
+except NameError:
+    def reversed(iterable):
+        """A 2.3-compatible implementation of `reversed`."""
+        lst = list(iterable)
+        return lst[::-1]
+
 # rpartition is new in 2.5
 try:
     "".rpartition
@@ -66,14 +75,16 @@ except NameError:
     range = range
 
 # A function to iterate listlessly over a dict's items.
-if "iteritems" in dir({}):
+try:
+    {}.iteritems
+except AttributeError:
     def iitems(d):
         """Produce the items from dict `d`."""
-        return d.iteritems()
+        return d.items()
 else:
     def iitems(d):
         """Produce the items from dict `d`."""
-        return d.items()
+        return d.iteritems()
 
 # Exec is a statement in Py2, a function in Py3
 if sys.version_info >= (3, 0):
@@ -129,6 +140,19 @@ if sys.version_info >= (3, 0):
         """Convert bytes `b` to a string."""
         return b.decode('utf8')
 
+    def binary_bytes(byte_values):
+        """Produce a byte string with the ints from `byte_values`."""
+        return bytes(byte_values)
+
+    def byte_to_int(byte_value):
+        """Turn an element of a bytes object into an int."""
+        return byte_value
+
+    def bytes_to_ints(bytes_value):
+        """Turn a bytes object into a sequence of ints."""
+        # In Py3, iterating bytes gives ints.
+        return bytes_value
+
 else:
     def to_bytes(s):
         """Convert string `s` to bytes (no-op in 2.x)."""
@@ -138,6 +162,19 @@ else:
         """Convert bytes `b` to a string (no-op in 2.x)."""
         return b
 
+    def binary_bytes(byte_values):
+        """Produce a byte string with the ints from `byte_values`."""
+        return "".join([chr(b) for b in byte_values])
+
+    def byte_to_int(byte_value):
+        """Turn an element of a bytes object into an int."""
+        return ord(byte_value)
+
+    def bytes_to_ints(bytes_value):
+        """Turn a bytes object into a sequence of ints."""
+        for byte in bytes_value:
+            yield ord(byte)
+
 # Md5 is available in different places.
 try:
     import hashlib