Adds blocking file locks.

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 4ac4486f6795ecda472864baa1e090da77d8b8da..92d144f941e48ca2e05e4ad0251c053549d90366 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -52,7 +52,11 @@ enum FileLock {
   /// Shared file lock.
   SHARED,
   /// Exclusive file lock.
-  EXCLUSIVE
+  EXCLUSIVE,
+  /// Blocking shared file lock.
+  BLOCKING_SHARED,
+  /// Blocking exclusive file lock.
+  BLOCKING_EXCLUSIVE,
 }
 
 /**
@@ -735,6 +739,11 @@ abstract class RandomAccessFile {
    *
    * To obtain an exclusive lock on a file it must be opened for writing.
    *
+   * If [mode] is [FileLock.EXCLUSIVE] or [FileLock.SHARED], an error is
+   * signaled if the lock cannot be obtained. If [mode] is
+   * [FileLock.BLOCKING_EXCLUSIVE] or [FileLock.BLOCKING_SHARED], the
+   * returned [Future] is resolved only when the lock has been obtained.
+   *

[siva, 2016/06/13 15:56:08]

This operation is not synchronous, should we allow the BLOCKING options to be
used in this function as a call to this function can potentially block and seems
to go against the grain of being asynchronous.

[zra, 2016/06/13 16:03:33]

This call is implemented as a request sent to the IO Service thread, which then
makes the potentially blocking call. The calling thread doesn't block. The
returned Future is completed when the response is received from the IO Service
thread.

[Søren Gjesse, 2016/06/13 16:05:09]

The blocking I/O call is dispatched on the "native isolate" thread-pool, so it
will not block other Dart code from running. However performing blocking lock
calls on more files than there are threads in the native isolate can end up
blocking the native isolate is neither of these calls can gain the lock, so care
should be taken in that case.

I cannot remember how many threads are allocated for the native isolate.

    * *NOTE* file locking does have slight differences in behavior across
    * platforms:
    *
@@ -768,6 +777,11 @@ abstract class RandomAccessFile {
    *
    * To obtain an exclusive lock on a file it must be opened for writing.
    *
+   * If [mode] is [FileLock.EXCLUSIVE] or [FileLock.SHARED], an exception is
+   * thrown if the lock cannot be obtained. If [mode] is
+   * [FileLock.BLOCKING_EXCLUSIVE] or [FileLock.BLOCKING_SHARED], the
+   * call returns only after the lock has been obtained.
+   *
    * *NOTE* file locking does have slight differences in behavior across
    * platforms:
    *