Corrected spelling mistake from "happend" to "happen" throughout multiple files.

pkg/dev_compiler/tool/input_sdk/lib/io/file.dart

Index: pkg/dev_compiler/tool/input_sdk/lib/io/file.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/lib/io/file.dart b/pkg/dev_compiler/tool/input_sdk/lib/io/file.dart
index 4ac4486f6795ecda472864baa1e090da77d8b8da..de526ed2f2eee568e2e3f975cd39a28a46567bab 100644
--- a/pkg/dev_compiler/tool/input_sdk/lib/io/file.dart
+++ b/pkg/dev_compiler/tool/input_sdk/lib/io/file.dart
@@ -582,7 +582,7 @@ abstract class RandomAccessFile {
    * bytes will be filled into [buffer] from at index [start], otherwise index
    * 0. If [end] is present, the [end] - [start] bytes will be read into
    * [buffer], otherwise up to [buffer.length]. If [end] == [start] nothing
-   * happends.
+   * happens.
    *
    * Returns a [:Future<int>:] that completes with the number of bytes read.
    */
@@ -593,7 +593,7 @@ abstract class RandomAccessFile {
    * present, the bytes will be filled into [buffer] from at index [start],
    * otherwise index 0.  If [end] is present, the [end] - [start] bytes will be
    * read into [buffer], otherwise up to [buffer.length]. If [end] == [start]
-   * nothing happends.
+   * nothing happens.
    *
    * Throws a [FileSystemException] if the operation fails.
    */