Document --analyze-all option.

dart/sdk/lib/_internal/compiler/implementation/dart2js.dart

Index: dart/sdk/lib/_internal/compiler/implementation/dart2js.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/dart2js.dart b/dart/sdk/lib/_internal/compiler/implementation/dart2js.dart
index c653e2f68a354518adfd740e5cdf2106dc4eeb27..55a016e10ea5ef35b8dfe5fa670a2d4b181b5ff0 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/dart2js.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/dart2js.dart
@@ -375,6 +375,12 @@ Supported options:
   -p<path>, --package-root=<path>
     Where to find packages, that is, "package:..." imports.
 
+  --analyze-all
+    Analyze all code.  Without this option, the compiler only analyzes
+    code that is reachable from [main].  This option is useful for
+    finding errors in libraries, but using it can result in bigger and
+    slower output.
+
   --minify
     Generate minified output.