Fix fuchsia toolchain prebuild setup

* Adjust bootstrap to provide useful output on failure
 * Add missing package dependencies in the build environment
 * Fix permission bits on prebuilt toolchain files

src/bootstrap/sanity.rs

@@ -221,8 +221,9 @@ $ pacman -R cmake && pacman -S mingw-w64-x86_64-cmake
     let run = |cmd: &mut Command| {
         cmd.output().map(|output| {
             String::from_utf8_lossy(&output.stdout)
-                   .lines().next().unwrap()
-                   .to_string()
+                   .lines().next().unwrap_or_else(|| {
+                       panic!("{:?} failed {:?}", cmd, output)
+                   }).to_string()
         })
     };
     build.lldb_version = run(Command::new("lldb").arg("--version")).ok();

src/ci/docker/dist-fuchsia/Dockerfile

@@ -1,17 +1,21 @@
 FROM ubuntu:16.04
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
-  g++ \
-  make \
-  file \
-  curl \
-  ca-certificates \
-  python2.7-dev \
-  git \
-  cmake \
-  ninja-build \
-  sudo \
+RUN apt-get update && apt-get build-dep -y clang llvm && apt-get install -y \
+  build-essential \
   bzip2 \
+  ca-certificates \
+  cmake \
+  curl \
+  file \
+  g++ \
+  gdb \
+  git \
+  libedit-dev \
+  make \
+  ninja-build \
+  nodejs \
+  python2.7-dev \
+  sudo \
   xz-utils \
   unzip
 

src/ci/docker/dist-fuchsia/build-toolchain.sh

@@ -27,7 +27,7 @@ git reset --hard FETCH_HEAD
 
 # Download toolchain
 ./scripts/download-toolchain
-chmod +x prebuilt/downloads/clang+llvm-x86_64-linux/bin/*
+chmod -R a+rx prebuilt/downloads/clang+llvm-x86_64-linux
 cp -a prebuilt/downloads/clang+llvm-x86_64-linux/. /usr/local
 
 build() {